I agree with half and half of whats been said already,
however no questions were asked, and straight asumptions were made, so I shall hit square 1 for the time being...
A - Is the PC a self built (assuming your not a hardware guy like myself I shall almost assume thats a no) if not then is it a prefab system built from the likes of DELLete (not as in elite but delete as in do not bother going there for consumer computers-business hardware is a whole different ball park) if you had it made from a local pc shop this still counts as a prefab... did you have a friend make it for you? or was it a hand down/buy off of someone/bought from ebay etc...?
B - Did you try anything said already?
C - If B = Yes and Solved = Yes - GO TO --->Do not bother reading further...
(hahahaha go read the anti anti go to thread for the joke there in C)
D - I forgot what I was going to write here after writing so much in A so just feed back more detail and perhaps a pic of the pc either open case if you have opened it before or if its a clear panel that would be ok, need an idea of the space inside, sounds like you got a DDR3 system as DDR2 is overkill for 12gig, DDR3 is cheap as chips atm...
E - How long have you had it for? Do check your warranties... hope you kept your receipts and packaging if any...
F - GL
G - Hope this helped, Over and out (for now)
EDIT
Corrected Typo
Also could you give GPU Specs and Motherboard model and BIOS revision if possible, also your PSU make and model too, (a whole system spec wouldnt hurt to diagnose any inferior hardware more likely to be more guesswork lol not that this is my point)
Vista... 32bit? 64bit? Starter? Home Premium? Professional/Enterprise? Ultimate?
(I know the 32bit/64bit question is rather dumb noting you have 12gig of memory making the assumption that you have a 64bit OS installation a default... but that doesnt mean you might not be running 32bit lol - Believe me I do come across this with some clients that bring their pc's to me for repair and I find this being the case regulary, (example 8GB in a pc running 32bit software...why o why... oh well makes for fun conversation for me)