I have done this twice to my computer. Be certain that you want to remove EVERYTHING from your hard drive.
Record and ensure you have all license codes, passwords,, etc. for ALL your programs!!!!!!
Purchase "wipe drive". This CD ROM erases everything on your computer, leaving it a totally blank slate.
http://www.datatexcorp.com/html/wipedrive.htm
If you google, I'm pretty sure there are some free versions to download out there.
This does take some time to do, and be sure to run it more than once.
When this is completed:
You need to re-install your operating system first and foremost. Be sure you have your license # from the CD saved somewhere. Again, this computer is now a totally blank canvas, and there is no way to recover all your license codes.
Re-install your PC's programs with the recovery disks.
Re-install your internet software.
Download and install all windows updates.
Re-install non-microsoft programs.
ie. virus protection, games, etc.
This should about do it. It is quite an undertaking, but it is like a whole new computer again when you finish!
Good luck!