Don't you have restore CDs, or a restore partition? Maybe install Ubuntu?
Otherwise, let's see. I would uninstall any programs that you purchased and plan to keep, then create a new "admin" user, log on as that user and delete every other user account. Go to "C:\documents and settings" and make sure there is only and "admin" folder and "all users" folder (may also be a "default" folder), delete any other folders. Check the root of your C: drive and make sure there are any straggling files or folders (other than "Program Files" "Windows" and "Documents and Settinsg" and the system files) that may contain personal information. Remove any dial up accounts, remove any wireless settings... Run sysprep.