You'll need: A Windows CD (XP 7 OR Vista) burned or not, doesn't matter... And a keyboard that works at bootup... and some basic knowledge
First, have the CD of WINXP, or the new OS, 2 restart the computer and press the bootmenu key on the first splash screen, 3rd use the arrow keys and open the CD drive option, 4th let that load and when it say Press any key to contenue, press any key... You should get a Blue screen loading files and then it will say windows is starting up, after that, at the screen, you'll see 3 options, choose the option you want (I think it's delete a partition) and after that you'll get a agreement menu, press F8, and you'll see a screen with all your partitions, select NTFS or C: and press D, after that press enter, now you should have empty partition space, Prss enter on that space, and you'll see the option to install XP on that partition, press the button it says to press and then select Install windows XP (not the fast one) and let it install, there you go, new OS