It's 1 of 2 things. 1. your running out of Vram, in which case you'd mostly get a DX issue/error. It's most likely 2. Running out of system memory.
Here's how you can pretty much solve this, save a few 100MB of Vram and System memory.
Right click the game your having issues with, right click the .exe
Go to properties
Tab over to the Compatability tab
Check the box where it says "disable desktop composition" and hit apply and ok.
It'll change your windows theme setting to basic or, non aero setting. It'll change back to your win aero setting after you close the game. This should resolve your issue. Hopefully your comfortable running in the basic theme while gaming. It's not a big deal but sure saves alot of resources.
Glad to help!