If Windows XP you can remove all personal information by deleting the UserName folder under the C:\Documents And Settings\
I would seriously consider getting this done by a pro. But, even better is to keep the hard drive. Hard drives aren't terrible expensive and it is worth you piece of mind if you are selling the computer to just put a new one in or to let the buyer know he needs to buy a new hard drive. It is possible to wipe the hard drive so that the information can never be recovered but it is somewhat complex and merely deleting all flies and reinstalling Windows isn't always enough. Although that will suffice to keep most users from accessing your past files. The very safest way is to keep (and physically destroy ie sledgehammer or fire) your hard drive. Hope this helps.