Under Virtual Server and Virtual PC you can enter the virtual machine BIOS configuration and then change to the Security page.  Here you can configure a supervisor and a user password.  You can then configure whether users can access the BIOS, view the BIOS or even be allowed to boot the virtual machine.

This functionality is also included in most physical computers, but most people don't use it.  This security can be relatively easily circumvented by just creating a new virtual machine and attaching the virtual hard disks to it.

Cheers,
Ben