Won't boot in safe mode

Hi,

I can't get my PowerBook to boot in safe mode. I hold the shift key after the startup chime, then release it when I see the gray apple and progress spinner; but the spinner spins for a while and eventually the computer shuts off. I can boot up normally, but I'm trying to boot in safe mode to rectify a problem I have updating Office 2004. I tried zapping the PRAM, but that didn't make a difference.

Thanks,
Michael