most cases will fit ATX, and microatx these days, but yeah, you'll want to look for one that says ATX formfactor. what kind of processor are you running? Make sure that the case you choose will fit your cooling needs also, if it comes with a power supply make sure that it is what you need, like the wattage.
When all else fails...Ham Radio!
I still have the same old HP DV9700T laptop I've had for years...still kickin'