You can restore most off the shelf pcs easily by putting in the xp disc or pressing a key that it says upon boot. But doing this you are liable to lose all your files, some restore programs at boot do however give you the option to use windows xp system restore from there.

Having never been a limited account myself, can you check if you can use system restore (via start > accessories > System Tools) on the guest account?

Oh right two different lynns using same computer.