Virtual PC / Virtual Server handle the mouse by emulating a standard two button PS/2 mouse with a scroll wheel.  Recently a number of mice have been coming out with 4th and 5th mouse buttons.  These buttons are usually mapped to the 'forward' and 'back' options in web browsers.  While it is not possible to have our emulated mouse support these extra buttons you can get the same behavior in a virtual machine by configuring these buttons to use the keystroke combinations of 'Alt + left' for backward and 'Alt + right' for forward:

Intellimouse control panel under Virtual PC

This will allow these buttons to work both on the host and guest operating systems - and on most current web browsers.

Cheers,
Ben