Bit hard to offer any help without any knowledge of your system configuration. Can you update your system specs. What type of computer, what graphics card, etc.
What resolution are you trying to run the game at, how much memory is on your graphics card?
thanks
Anyway :
Using Windows 7 64 bit 4GB RAM intel core i5 (4 CPU) 650 3.20 GHZ, Video Card is ATI Radeon HD 5670, System Manufacturer : Gigabyte Technology, using DirectX 11
updating the Video Card will help?
I'd start with making sure you have the latest drivers.
The card seems to be an old card (released in 2009), at the entry to mid level, looking at various websites. Over time games become more demanding, both in GPU requirements and power requirements.
This post should probably be moved to the gaming sub forum.
Upgrading your card might be a worthwhile option if you want to continue to support new games. You didn't say if you have the 512mb or 1gb RAM model of the card. If you have the 512mb model, that would help justify an upgrade if you wish to continue to play new games.
Are you running any screen management or windows explorers shells or replacements, like Desksoft's WindowManager? This has been known to cause problems with some programs running full screen/windowed or minimised.
Can you pinpoint any system change at which the problem started occuring? For example, did you install any new program? If so, did you try a system restore to before that?
Have you got DX9 installed? Some programs need DX9 as well as DX11, though you will usually get fatal errors in those games if DX9 is missing? Are you seeing any error messages?
Have you tried lowering the resolution the game runs at, playing with with other various settings?