To be sure, you basically need to wipe the disk and (assuming you're selling it as blank but with Windows installed) reinstall the OS.

If you don't know exactly what you're doing and don't want stray personal details left on the thing, then the best bet here would be to find a local Linux geek and ask them personally to (and I quote) use a Linux boot disk to "dd if=/dev/zero of=/dev/hda" to wipe the disk, then reinstall Windows using whatever CDs you got with the machine. Make sure you've moved anything you *do* want to keep off before this though; only the military get to recover data after that sort of destruction!

Thanks, but I didn't want to be that harsh. I just wanted to know an easy way to destroy those hidden files without messing up the machine. (as most programs I have no CDs, or access to them anymore)

I don't think I'll be able to wipe the disk, if I wanted or not.