I had my PC uncased earlier this year, after Xena's Guy upgraded my video card.
Then we put the case back on, for what were likely aesthetic reasons, and now one of two things happens - almost always while I'm playing "City of Heroes."
Either the PC gets too hot and reboots, or Dennis the cat sits on top of the PC and blocks the vent, and the PC gets too hot and ditto.
There is seriesly nothing more frustrating than being in the middle of a pitched battle against space Nazis and other forces of evil, and having your damn screen go black . . . and then that Windows start-up logo shows up.
Grrrrrr.
hehe, I know your pain! Except mine never overheats. I just keep the side off of the case because I'm always inside there messing around with stuff! :)