Like Bruce, I'd try a good clean first. I had the same problem last summer (not THAT hot in the UK!). I downloaded a freeware product called CoreTemp - see http://www.alcpu.com/CoreTemp/ and was fairly astounded to find that both cores of the processor were running at well over 85 degrees centigrade.

I had a look at the fan on the side of the case and this had quite a lot of cat hairs in the mesh (Birmans and computers don't mix). There was also a lot of dust round the processor board and fan. Easy to clean with the crevice tool on the vacuum cleaner, but make sure the box is powered off and disconnected first, also don't knock any connectors!

I had another look at CoreTemp afterwards and the both cores were running closer to 32 centigrade.