Open up the Virtual PC Window which shows your virtual machines (".vmcx" files).
Right-click on a Virtual machine and select "settings".
Click the "Integration Features" setting in the left panel, then UNCheck the "Enable at startup" box in the right panel.
This will give you a console window that shows the BIOS as the machine starts up. Start the machine, and quickly hit the DEL key in order to get the BIOS configuration menu.