My computer won't boot!

Hello,
Hi can anybody help??? My computer keeps telling me it has shut down unexpectley! It gives me the option for safe mode ect, but my keyboard won't work in that screen even if I use f8! I know the keyboard works because it lets me go into the system setup etc! But when I'm in the safe mode screen it dosent do anything! It counts down then goes no signal and starts back in the Same screen again!! Help???

Windows xp
Thanks