Hmm,..well maybe too a high polygon count..i/e detail..I'm just guessing here now..but I have noticed that in my rendering at times. You might check and make sure too..there isnt a lot of other stuff running in your task manager hogging up memory like those pesky svchost things.. a big hog too is Firefox..so dont go googling while you render..

Really though..most times I do renders..I just back up from the pc..let it go..and just go take a nap.. LOL