For starters, physical access is total access.
If you give someone your laptop, they can use a ntpasswd boot cd to gain access to your Administrator account. From there, the sky's the limit.
Other than that,
A Guest Account:
- Cannot install software or hardware, but can access programs that
have already been installed on the computer.
- Cannot change the
guest account type.
- Can change the guest account picture.
They don't have rights to install new software or printers, though they may be able to install software that doesn't require traditional installation like Chrome.
If you're really worried about them making changes, you can always use a program like Deep Freeze to prevent anything from permanently changing. Though, with unsupervised physical access to the machine, there are ways to get around it.