The best way to wipe the hard drive of a PC is to use erasure software. We recommend Darik's Boot And Nuke (DBAN).
DBAN will wipe and overwrite your drive several times so nothing is recoverable. It secures your personal information by making it impossible for anyone to recover the deleted information from your old computer.
To use DBAN, download the file and burn it to a blank CD or DVD. If you have any questions, visit the site's FAQ http://www.dban.org/faq for answers. Once the disc is done, boot your computer from it.
To boot from a disc drive, hold down the F8 key while your computer is starting. This brings up the boot menu. Then select to boot from your CD or DVD drive. Once DBAN starts, follow the directions.
DBAN securely wipes out the entire hard drive. When it's finished, the computer won't even have Windows. Then you can safely discard, sell or recycle the computer.