I had the same problem. It is from a failure of winsat.exe (the engine computing the Windows Experience Index (WEI) in Win7) to play the file Clip_1080_5sec_10mbps_h264 in the Windows/Performance/WinSAT folder (the other clips in the folder play fine for me). This file is played during the WEI phase "Tuning Media Decoding". When it fails, the computer appears to lock up, but it really still running, updating the desktop at about 1 frame per second. This makes if very difficult to move the mouse or to get out of windows even by using the ctl-alt-delete method. But it can be done if you are patient enough to do so. Or you can try and wait out the 600 second deadman timer that weisat.exe uses to bail out. After 10 minutes (or so), the graphics interface is still extremely slow, so you have to get out and reboot to clear it.
I have exactly the same problem with either the ATI 4670 or new ATI HD5850 graphics card. It occurs with the Win7 RTM Home Premium drivers (in the case of the 4670) or the latest Win7 catalyst drivers from the AMD (ATI) site for both the cards.
AMD.com | Support & Downloads
I agree with the above posts that it is probably an issue with the Gigabyte motherboard driver, as I ran WEI fine on a different motherboard, Core 2 CPU E7400 and DDR2 memory.
In my case, after countless hours of swapping hardware, reinstalls of various Win7 software, the only solution I have found with the GA-P55M-UD2 motherboard setup, is to eliminate the Win 7 Aero mode by using a non-Aero Theme. To do so, right Click on the desktop, select "Personalize" and pick a Basic and High Contrast theme - like Windows 7 Basic.
You will lose the AERO features, but WEI will run - at least on my machine!
Hopefully either an update to Win7 or the appropriate drivers will resolve the issue soon.
