have a look in event view 9honest not my answer to everything but a good place to start
)
event viewer
start -> settings -> Control panel -> Administrative tools - Event viewer
(or start run eventvwr)
that's XP
Vista it looks different, but in a similar place!
Could be a hardware or software reason for it not shutting down. Ticking could be hard disk (event viewer may suggest that) or PSU , which event viewer wouldn't really no about but would explain starting up problem.