Well, your machine shouldn't be generating any more heat than it used too...a machine generally generates a set amount of heat (for the task, dependent on CPU load, etc), and then it's about how well or how poorly it dissipates that heat. Lots of heat inside machine = bad.
As a matter of fact, a really good cleaning would, if anything, make your room hotter since more of the heat is making it out into the ambient environment instead of being trapped in the box.
So, if your room is hotter...are you running more processor-intensive tasks lately? Something like Folding@Home or BOINC? Or maybe your power supply is having some kind of trouble that is causing it to overheat? I don't know much about power supplies though...