After saving everything put the windows cd (or whatever operating software you use) in the disc drive. Reboot your PC, and let it boot from the cd. There are multiple ways to archive this: press (I think, not sure) F8 while the computer is booting (so before the windows splash screen) and select 'boot from cd drive' or something. Or you can go to your system bios (be careful with these options though) and set your PC to boot from the cd drive. Or, some PCs automatically check if there is a cd in your drive, this depends on the boot priority settings in the bios.
Anyways, let your pc boot from the operating system cd. Carefully read the settings, and let it reformat (long) your hard drive(s). Then the PC will move on with installing the OS.
After installing your operating software, you can use your pc again. You will find a nearly empty desktop.

Install your graphic card drivers, sound drivers, etc, all the drivers that came on the cds with your pc.
This should do the trick. Use a rainy sunday for this, as the complete process including the reformatting may take a long time.
Good luck
