I'm too lazy to dig up the thread, but this is apparently a driver bug. Just keep the affinity to 1 core. There is a work around here somewhere for it....
I tried very hard to fix this problem on my one windows rig.
I ended up unlocking the second core of my Sempron processor so the rig would be usable as a desktop.
If you have multiple cores/processors:
1. start the miners
2. go to the task manager (ctrl+shift+esc)
3. in the processes tab right click each miner process and click "Set Affinity..."
4. put all of the miner processes on the same cpu