I have a quite old graphics card: a 9400GT. When I had Windows XP installed in my PC, it worked pretty well for the whole time. I was even able to play GTA IV in the worse conditions possible - everything settled to a low resolution -. I could also play not so "heavy" games like Portal, Left 4 Dead, GTA San Andreas and so on.
Then, like 8 months ago, I installed Windows 7, and as I realized, that kind of problem doesn't happen only to me, but to everyone else who have an old graphics card and Windows 7. The problem is: when I start up a game, it runs perfectly for 5 to 10 minutes, depending on how much memory it consumes. If I try to run GTA IV, it runs nicely for 3 minutes. If I try Super Meat Boy, it takes 10 minutes until the problem happens. After these minutes, the game starts to slow down until it reaches 2~5 fps. And as I said, it happens not only for 3D games that need a better computer, but also to 2D games such as Super Meat Boy and The Binding of Isaac.
Probably the problem is on the temperature that my video card reaches when I start to play a game. It's something about 90~110 ºC. I don't know why it's overheating now, after so many years using it without any issue. Does anyone know what could be happening to it and how I could fix it? I'm going to buy another one soon, but I'm looking for solutions for weeks and I still didn't found any. I've already cleaned up my CPU and it didn't help. I've also tried to change some values in the Windows Registry but it didn't work.
I'm not sure if the only problem is the overheating. The only thing I didn't tried yet is to put a fan in front of my CPU, but I'll do it later and see if it helps. But if anyone knows what else I could do, I'd be grateful.

My system specs:
Intel Core 2 Quad Q6600 @ 2.40GHz
4GB RAM DDR2 @ 333MHz
ASUSTeK Computer INC. P5K SE
1024 MBGeForce 9400 GT