You can use any install disc with your key
As long as you have the key for that computer, you can borrow or copy an install disc from someone who has one, then wipe the hard drive with something like DBAN followed by reinstalling Windows. That could take the better part of a day to do, but it's the safest.
As an alternative, you could delete the contents of your Documents folder and any other folders you have sensitive data in, then do a defrag. This is subject to your missing something and doesn't always write over 100% of everything you deleted, but it's probably good enough for the majority of folks who don't have super-secret or incriminating data on that computer.