It SHOULD be F8, But you could try:
Turning the PC on, and when it gets to the XP/Vista loading screen, yank the power cable out (could be bad for the pc......). Next time you fire up the PC, it should take you to a menu saying "Windows failed to load... yadayadayada. Choose option 1 to go to safe mode" (or somthing similar)
Using a PS/2 Keyboard, as the drivers might not be loaded (this would be strange, as the BIOS normally controls the keyboards input right up to a sucessfull boot)