Lior, out of interest are you sure your leaving the render long enough to render? 

Sometimes even on a quad core the computer can appear to lock up, then releases itself a few mins later. It might be worth leaving the render for 30 mins and then come back and see what is going on, if anything.

Also render to screen so you can see that something is actually happening.

Jon

DA Portfolio - http://jonj1611.daportfolio.com/