If you`re talking about what I think you`re talking about...then that disc should set your PC back to the status it was when you bought it. IE Windows pre loaded with all the software etc. they gave you.
Anything you have installed since then, plus all your documents/settings etc. would get wiped out - so be sure you have thought about backing all that up (post for details of anything you don`t know how to backup) before you attempt it!
Miles