Could someone help me please.. Startup problems

Hi.
My macbook keeps freezing when ever I try to boot it up. It's get to the apple logo with all spiny cog thing, and then it freezes. I tried to open up the help menu, but it did the same thing. Anyone got any ideas on what to do or is it a case of i'm gonna have to whip the harddrive?

Thanks

I've tried all of those. and none of them seem to work.
safe mode freezes.

I'm also using an external hard drive as my hard drive, i dunno if that makes any difference. and it's mountain lion.