I'd start by checking how much memory your machine detects vs how much you are supposed to have.
Assuming you're running some variation of Windows just right click on "(My) Computer" (either on the desktop or in the start menu) and go to "Properties" and it should list the current operating system and basic info about your processor and memory.
If that value is not right you probably have a faulty DIMM (stick of memory) and you'll need to either re-seat it and try again or replace it entirely. If it's the right value then I suggest a restart. If that doesn't fix things I guess it's time for an upgrade or a windows reinstall!
If you're on a Mac then I'm not of much use. Sorry
P.S. I completely overlooked what Barozi mentioned. Rookie mistake by me :). You also don't seem to be having much luck with your gaming hardware lately. Sucks!
