Reformat Win7?

Hey, I have been having lots of problem with my computer. Here is a couple of examples:
- Lots of space used.
- Errors.
- Firewall problems
- and others....

I haven't reformat/re-installed my windows 7 in 2 years, so I thought I should do it now since I have couple of problems. But I forgot how do you reformat your PC? How do you install Windows 7?
I know that you have to buy windows 7, but is there a free way? can you pirate it? What's wrong with getting a copied windows 7? Really be helpful if you guys can tell me more about reformating my pc since I didn't do it in couple of years now. Thanks =)

You could Try Deleting Your Temp File's And UnInstalling Useless Program's.
Running Ccleaner
Run AV Test's
Malware Test's

You reformat your PC by doing a Clean Install of Windows 7... Booting From Windows 7 Disk/USB Flash Drive and reformat hard drive from there...