It's probably the power supply, my old Antec NeoPower 480 used to dump an unbearable amount of heat into the room. No joke, it had at least a 10 degree impact on the room temp. When I changed it out for my current Enermax 600W, all of that disappeared. There's no noticeable impact anymore (I'm sure it's a couple of degrees, but not immediately noticeable) in room temp between the pc room and any other room.
I ended up giving that Antec NeoPower to my nephew, his old psu was a no-name $10 350W model. The 350W did not heat up the room but sure enough, as soon as I installed the Antec it became unbearable again. I eventually swapped it back out for the generic 350W unit just to give the kid a break.
So check your psu, see if it's dumping a ton of heat out the back. Mine sure was, but my Enermax always blows semi-lukewarm air instead of the Antec's skin melting, scorching hot air.