You guys are giving some horrible advice. Simply disabling the guest account is not the same as deleting it. One of the fundamentals of network/computer security is removing nonessential accounts and services. I don't normally work with windows, but will start looking for a way to remove the guest account.
Edit: I was a bit harsh. All the advice you guys gave was good, except when you told him not to try deleting the guest account. As far as limiting users' rights, and installing a BIOS password, that is all great advice. I'd still try to find a way to delete the guest account if possible.