macbook pro late 2008 won't boot

Hi,

I have a macbook pro late 2008 and every time I try to turn it on I see the Apple logo and the loading bar fill up partially, stop, then it reboots itself again and again and again. It gets to a certain point in the loading cycle and restarts itself.

I can't find my original OS X install disc. I've researched a bit that maybe I could try booting to that. I'm not terribly worried about keeping the data on my computer - more worried about getting it to function period!

Any help or advice on where I should start would be greatly appreciated!