You need to set their accounts as limited. They wont be able to activate the guest account or change anything on the admin account.
The docs I was talking about were off the microsoft knowledge site at microsoft.com
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.
And the reason why deleting my guest account was so important is because I used it as my primary user account with all my emails, files etc for the last 3 years. Now there are a few 'real' guests in my house and I do not want to risk exposing all my files and emails to them so I have moved on to another password protected full user account.