I've been known to have a similar problem, but it resolves itself after reboot.
I found it occured after a system hang, an update, a lost internet connection, or an msn messenger freeze. I couldn't open IE, Control panel, network places, any explorer or search window...
It hasn't happened for a while now, so it seems to be fixed, although I did nothing to fix it.
Just posting in case this might shed any extra light on the problem.