Hmm, what is the usual heat temperature where you live? Because that can be a factor you know.
Personally, I live in the UK, and with our weather, I've never had a problem like that, has anyone else had overheat problems, or the mchine is so cold it can't operate?
If it overheats, then open it up and add another fan to the side (I beleive one of those old overclocking fdans for the frst pentiums should do the trick, VERY Powerfull and large) or if it's too cold, well, try and heat it up.
If you try to run a fan from the internal power supply, you may do more damage than good. Depending on the current draw of the fan, you may eventually fry the supply by forcing it to draw more current than it was designed to handle.
The overheating may be caused by a dust build up inside the case, which could be easily fixed with a vaccum cleaner. More likely you've got a faulty component somewhere on the supply board itself - get an electronically minded friend or technician to look it over for suspect components.