If you have misplaced your restore CD then you should be able to get a copy from the computers manufacturer. If not then you need to get a copy of Windows and all the drivers for your computer, format the machine (you can do this from the XP Installation disc) and do a clean install.

I would, however, take the advice of ChatMod#1, if your not sure on what your doing and use cleaners to get rid of all the junk on the machine. You will notice the difference and wont have any of the hassle of finding drivers and the expense of buying a copy of XP