A few times lately, my shortcuts won't open their associated programs. Double clicks or Right click - Open produce the annoying circle that's supposed to be friendlier than an hourglass. Going into the Programs listing of the Start button won't open them either. Meanwhile within my browser window or other currently running programs, everything responds fine.

If I reboot, things return to normal.

What is going on? (I have virus and spyware and firewall protection.)