Well when I had similar issues, I merely had my super darling husband, who is massively technical minded, add a better heatsink to both the video card and the processor... Which solved the issue. In the case of my video card he also mounted an extra fan to help move the air and cool better. I stand corrected he just informed me that he put a more powerful cooling fan in later since the extra one [while helpful] wasn't cooling enough either.
Another thing that makes a big difference for me is weekly defragging of the drive and cleaning junk files from the hard drive. I use Crap Cleaner.
www.ccleaner.com I like crap cleaner because it is simple, thourough, plus I have never had an issue where it has messed things up like some cleaning programs I have tried in the past, and best of all its free. Due to the amount of web surfing and use my computer gets, I run Crap Cleaner daily, sometimes more than daily.
On the subject of cleaning, make sure the air intake/outflow areas of the computer are free of dust and debris since if it cannot breathe it cannot cool down properly even with heatsinks and fans.
Also it is still a good idea to run the windows based disk clean up program to clear the 'old compressed files' monthly
some other things you can do [that I have also done in the past] is to decrease the visual effects for windows [especially in Windows XP] while playing, and (I know no one likes to do this) on the games that have the most trouble with it overheating run them at lower settings.
IF you have had the computer a while and this has recently become an issue I strongly recommend hunting up a new video card before the old one gives out completely.