It even happens when aero is enabled? And are you using an aero theme for windows and not windows basic or windows classic themes? Because you do have to use the actually glass/aero theme for aero to be fully enabled. If aero is on, then I really am quite baffled as to why window capture would be doing that, I've never seen that happen before. I wish I had direct access to the computer to find out the exact cause.
The only other suggestion I could make is just to use monitor capture instead. Or, alternatively, try cropping monitor capture. If you're using it just for the lobby then you can ignore the framerate issue it has if you have aero on (just make sure to turn on "use CFR" in advanced).
Qutie strange though, I would love to spend some further time with the computer seeing what's going on some time.