This is what I would do.
Uninstall as many of the programs as you can that you added to the system.
Delete all favorites, cookies, saved passwords, and autocomplete history, email accounts, as already mentioned
Create a new administrator account.
Log in as the new administrator and delete the old administrator account and any other user accounts that exist.
Go into C>Documents and Settings and delete any remaining folders that have the name of the user profiles you just deleted.
The only way to save your XP install and zero write the hard drive would be to make an image of the your now basic setup, zero write and then restore the backup.
No need to be paranoid, it would be pretty strange for someone to spend a lot of time and effort trying to find something that they have no idea is even there in the first place, and hence don't know what they are looking for.