I honestly can't see how this virus can be active before windows is up and running, so I can't see how it can stop you interrupting the boot sequence. You need to keep pecking away at the F8 key like a demented woodpecker to catch it before it gets into a routine boot sequence.
The problem may well be hardware related, if you have a USB keyboard, which unless the BIOS/CMOS settings are set to recognise USB devices before the OS is loaded, it might not be recognising the keyboard input. If you have a ps2 keyboard you could plug that in and reboot and see if that allows you to interrupt the boot.
You could also check out this google search and see if there is anything listed that might help
restart into safe mode from the "command prompt"