I concur that you might want to take a look at the task manager's resources window. I was shocked the other day when a program (I forget which one) was eating up 4GB of memory all on its own for no apparent reason. It was something stupid like the Intel Turbo Boost program sucking all the RAM.

If I hadn't been running Process Explorer at the time, I never would have suspected anything. I just happened to notice its memory window was telling me that just about all of my 6GB had been allocated.