PC not booting - likely cause??

[Diss]

Hi

Woke up this morning, turned on PC and nothing! Well almost nothing. I get the first beep (it does actually start) but no bios screens then just a lot of HD activity and my monitor light flashes on and off.
Same happens if i try and boot from a live CD

Any idea what might cause this?
bad HD?
RAM?
mobo?
monitor?
vid card?
Other?


thanks

Diss