All games run fine, suddenly lag/stuttering?!

About 2 weeks ago I was running FIFA 13, Hitman Absolution, Dirt 3... everything was running smoothly, the fps were great and I played them fine. Now when I play FIFA 13 it's almost impossible, it stutters like hell, Hitman Absolution gets really low framerates and the experience just isn't the same anymore.
Basically all my games aren't running as they should. I reinstalled my video drivers, DirectX, did a virus scan, cleaned the HDD, cleaned the little dustbunnies inside the PC, nothing.
One thing I noticed today is that when I started Vegas Pro 11, an error came (Vegas Pro Has Stopped Working, send, don't send.. etc) up just after it started to initialize GPU-Accelerated video processing. Now I uninstalled my video driver from the Device Manager and after that Vegas Pro 11 worked fine but no games worked because it seemed that my PC needed a restart. I restarted, the video driver was back in place and Vegas Pro 11 didn't start again.
I'm not sure if I completely uninstalled the driver but that meant something? Right now I will go and try to find the disc that came with my video card: ATI Radeon HD 4850 1GB.
Other specs:
Intel Core 2 Duo E4700
Windows 7 Ultimate 32-bit