When you turn it on I think you have to press either alt u, or control u, or just u. One of those. It allows you to choose what system you want to run on (mine has both os x and 9.2), but also helps to start it up.
If that works, go into finder, search 'Disk Utility', and do a scan, so as to clean up anything nasty in the hard drive
I just realised something- itunes and quiktime don't require restart! A plot twist!