If you go into the 'Full-Screen Mode' of the Virtual PC Options section (accessible via 'File' --> 'Options' off of the Virtual PC Console) you will see a single checkbox - which is checked by default.  When this option is checked, putting a virtual machine into fullscreen mode will cause the host operating system to change its monitor resolution to the one being used by the guest operating system.  This allows you to see exactly what you would normally see if you were running the guest operating system directly on the hardware.

However - if you uncheck this box the host operating system will not change its resolution - and instead it will draw a black border around the virtual machine display when it is in fullscreen mode.  This can be useful if you want to run a virtual machine in fullscreen mode - but the resolution it is using is not supported by your monitor (or - in the case of LCD screens - it may just look a little odd).

Note - that either way, if your virtual machine has Virtual Machine Additions installed and desktop resizing enabled (which is enabled by default) the host operating system will not change its resolution and will instead resize the guest operating systems desktop to match the resolution of the host operating system.  If you want to stop this from happening you will need to go into the virtual machine settings and disable desktop resizing under the display section.

Cheers,
Ben