Windows - All of a sudden mouse and keyboard not working

I've had XP SP2 installed on a 20" Core Duo iMac for a few months and it was working fine.

All of a sudden, neither the keyboard or the mouse are working so it's completely unusable.

The keyboard doesn't work during scandisk either (ie. where you can press any key to skip disk checking) - so it must be running into problems early on in the boot process.

I've tried unplugging all USB peripherals apart from the mouse/keyboard, but it still won't work.

Any ideas?