cgminer's windows version has other bugs as well.. apparently it doesn't clean up properly when quitting it, try starting and stopping it couple of times and then your hashrate drops to about 10% of the max, only rebooting or disabling/enabling the video card from device manager helps at that point.
I've been having the exact some problem.
I usually just sign out and sign back in to make the issue go away. But temporarily disabling the graphics card from the device manager seems like a better option. Hadn't thought about that yet
