Read what is scary at http://forums.cnet.com/5208-6132_102-0.html?threadID=119247

So while you can scrub and clean files (I think CCLEANER from download.com is nice) if you read the first link here's the brutal truth. Windows goes to inordinate lengths to store useful information in the most arcane places. If there is a lesson here if there is something you don't want to leak from your PC you replace the drive or at least start over with DBAN (do not complain to me if you use that!)

So in short, read this is scary, clean out the stuff you find then create a NEW ADMIN ACCOUNT for the new owner. Then log in there to delete the old account PLUS delete the old user home directory. You're pretty clean at this point but run the THIS IS SCARY tool again and CCLEANER one last time.
Bob