Macbook OS will not start

i have a three year old macbook..
when i try to start the machine a grey circle with a line thru it comes on the screen...
the whirling disk keeps whirling but the OS does not start..
anyone know which buttons to press on startup to fix this problem?

G'day and welcome to the forums.

Try zapping the PRAM, boot and hold down Command, Option P and R keys all at the same time immediately after the chime, and keep held down until you hear the third chime.

No joy boot from your system disc and run repair Disk from there as you may have a failing hard drive.