Reduce CPU usage, speed up PC

Steps

1
Clean computer registry, but first backup the registry using the cleaner program. Regularly use a freeware[1] or purchase commercial, registry cleaner to increase XP performance. Maybe you have uninstalled an application without using the Add or Remove Programs function in the Control Panel, or perhaps an object or file in the registry got moved. Eventually, this orphaned or misplaced information accumulates and begins to clog your registry, potentially slowing down your PC and causing error messages and system crashes.
2
Remove Spyware and Viruses
Spyware and viruses are leading causes of computer slowdown because you may have several spyware cookies or trojans that are taking up your processor time for reading and storing and reporting your activity to several spy databases. Read Rid-Your-Computer-of-Adware,-Spyware-and-Viruses to learn how to remove them and speed up your system.
Update and run your spyware and virus software every week.
If you do not have tools installed to protect against malware and spyware then you can download Spyware Blaster, and for viruses, download a free copy of AVG - "Anti-Vir Guard" or this one Avira - "Anti-Vira guard" (all three of those are free "For Personal Use"), all three are also available in Business or Pro versions. Also, Microsoft's Windows Defender is a widely-used anti-spyware tool that is currently distributed freely for users with a "Genuine" verified copy of Windows. "Windows Defender" is also included with other Microsoft services and products such as "Live OneCare" and "Vista."
Download and install Mozilla Firefox, Opera or Google Chrome. It will allow you to import all of your Internet Explorer settings, and is much less prone to malware than Microsoft Internet Explorer. It will prompt you with a "Set as Default Browser" message. Check "Don't show this again" and click "Yes". Firefox also has a cool feature to delete cookies, cache, and temporary internet files each time you close it. You are strongly recommended to use this option. It can speed up your internet experience. You should also check Google Chrome which is noticeably faster and less prone to virus attacks from internet Chrome.
If you have a really complicated virus infection, try using the Quick Virus Remover. It is a free utility, though not a replacement for commercial products but can be helpful for removing some of the most common viruses infecting your system.
3
Perform a disk clean up.
Click start in the bottom left corner, then click run.
Type, "cleanmgr.exe" in the text field.
Press "okay". This can take a while if you have never done it before.
4
Remove unwanted/unneeded software which may be the reason for slowdown.
Click start, then click control panel.
Click add/remove programs.
Highlight any old software you do not use and then click "remove".
5
Defragment Your Computer - This will place similar files physically closer on the hard disk and will help improve load speeds.
Click start, again, then click run.
Type, "dfrg.msc" in the text field.
Click Defrag to start it.

6
Stop unwanted Start-Up programs.
Run msconfig
Choose startup box
Select uncheck programs you don't want to run on startup

OR

Click here and download StartUpCPL.
Install the software you have just downloaded.
Open control panel, select Start up.
Disable any programs you do not want load automatically.

Programs can also schedule tasks to launch automatically when windows starts to stop such programs use windows task scheduler.To open Scheduled Tasks, click Start, click All Programs, point to Accessories, point to System Tools, and then click Scheduled Tasks.
Disable All Services you do not use or need. Open control panel, open Administrative Tools, and select Services. Disabling the wrong Services may lead to serious problems that you may not notice right away. Please consult a reference such as overclockersclub.comfor information on specific Windows XP Services. If you do not know what it is, do not disable the service

7
Make Windows Load Faster
Click Start, then Run.
Type in 'msconfig' and press enter.
Click on the BOOT.INI tab at the top.
Over to the right there will be a box labeled Timeout with 30 in it. Change the 30 to a 3.
8
Speed Up Processing Time
Turning off the fancy Windows XP graphics will noticeably increase the speed of your computer.
Go to Start, (Settings), Control Panel, System. NOTE: You may need to 'Switch to Classic Mode' to see the System applet.
Go to the Advanced tab. Click the Settings button under Performance.
Select the 'Adjust for best performance' radio button, then Apply, and OK.
The graphics aren't as flashy, but your computer will run a lot faster.
9
Set Pagefile size
Go to Start, (Settings), Control Panel, System. (This is just like the step above this.)
Go to the Advanced tab. Click the Settings button under Performance.
Now switch to this Advanced tab, and click on 'Change' under Virtual Memory.
You will see the 'Initial Size (MB)' and the Maximum Size (MB)'.
Change 'Initial Size' to the same value as 'Maximum Size' then click set.
NOTE This setting will help your gaming process by speeding up your computer.
ADDED NOTE: When your computer consumes your computer's hard drive and uses it as RAM, it is referred to as thrashing. Thrashing is bad for your computer and buying a RAM (it's not that expensive) is much more worthwhile.
10
Set Priorities:Press Control (ctrl) Alt Delete (del) at the same time. Once task manager opens, click the process's tab. Now find explorer.exe, right click it, and set the priority to realtime. This makes your computer focus on explorer.exe, which is your task bar and visual objects. Use this method if you want to keep your visual styles (or your using a transformation pack). This method greatly increases speed and if you are noticing that other programs that you use a lot are slow, you can set their priority to either high or above normal, if you have 2 process's on high at the same time, your computer will be unstable, and might crash. **For users without Admin password**-You will not be able to modify the value to realtime, if you aren't on an admin account, or you have insufficient privileges, then the best you can do, is set to high.
11
Delete photos. Pictures can take up an enormous amount of space. If you want to keep the pictures - and you most likely do - copy them to a CD-rom.

edit Tips

If this is your first defrag, do it after the disk clean up twice, then a disk defrag, and then another disk clean up. It is suggested to defrag your hard drive weekly.
For best results, run defragmenter last after performing all other steps and do not use your computer while defragmenter is running.
Do all the above steps quite often to maintain the speed of your PC.
Run an alternate Windows shell to save on RAM usage and boost performance (Shell Shocked: Alternative shells for Windows). Although more expensive, installing additional RAM is a better option. Running an alternate shell is a compatibility risk as many proprietary Microsoft programs, such as their latest game or Visual Studio, may not function properly on a third-party shell.
Although time-intensive, a re-partition and re-format of the hard disk drive, along with a clean installation of Windows XP will improve performance. Performing this task will remove all of your existing data, so a back-up or file copy is recommended first. Only save the files you created, as you will need to reinstall each application on the new installation of Windows. If you did not change the default location when saving documents, it may be safe to copy only the user folder and all files and folders within it (ie: "C:\Documents and Settings\(your user name)"). Typically, most users will need to be sure to copy these files:

Documents created using applications such as Microsoft Word or similar office suite programs
Bookmarks/Favorites from your Internet browser
Fonts installed that are not included by default with Windows (note that some fonts are installed by applications).
E-mail in-box and folders if you use an e-mail client that does not use an Internet browser
Any schedule data for programs such as Outlook
Financial records for programs such as Quicken
Optimize the registry with a third-party application. Many of these registry tweaking programs are available on the Internet, including freeware, shareware and commercial applications that are released as free trials. If you are not familiar with the registry, do not try to edit it manually -- the registry is vital to Windows' operation.
Download Ccleaner from majorgeeks.com. It's a good freeware program, and if you don't use other unnecessary programs, you will be able to gain a lot of space. It also gives you other features such as:
Startup Manager and
Registry Cleaner.
Also go to Start/Run type in %temp% and delete them all.
Clean your Computer's Inside. Dust off the fans, gently wash the monitor screen, and clean the keyboard/mouse. Dust blocking the fans and heat sink inside the computer can cause performance problems, including memory problems.

EDIT:credit to BrettCapewell, Zach, Sondra C, Nathan Wong