Crap. Now try underclocking your memory to 300 MHz, maybe that helps...

Did you try leaving it alone(logoff or unplugmonitor or..) and calculate from "2^32 * share/s = hash/s" from your pool site(~3hours to get most accurate). I know there's some variance, but should clearly see if they are still running 50% or 99%.

Another idea, burn a LinuxCoin cd and try mining in their live environment. If it still stays low, we'll know it's probably hardware related.
http://forum.bitcoin.org/?topic=7374.0