Hit f8 as the system loads. you get the options for safe mode. If safe mode won't boot reboot and f8 select the don't auto reboot. You will get a blue screen. At the botom is an error code. Search that code and the source of your problem will be a bit easier to diagnose
I'm having the same problem only that it will only allow me to hit F2 & F12. I have tried hitting F8 but to no success :s. Any suggestions?
By the way I have a dual OS... Windows 7 and Ubuntu...