Windows 7 x64 - Explorer windows stop responding

I just did a custom clean install (formatted the old system partition) of Windows 7 64 Home Premium on a system that was previously running Vista 64 SP2 without issues. Then I installed a handful of applications, like 7zip, Avira Antivir, Foobar2000, VLC and Firefox.

Twice tonight, Explorer windows stopped responding (Windows Explorer, not Internet Explorer). I tried left-clicking on various different windows, on the the top edges, on the scroll bar, on the corners and on files in the window and nothing happened. I tried selecting other windows and the same thing - no response. I right-clicked on the Desktop and didn't get any menu.

The first time it happened, I hit Ctrl-Alt-Del and went out to the Task Manager, then I could click and switch to application and the windows seemed to go back to normal (clickable and movable as expected).

The second time it happened, I used the mouse to go down to the Windows start button and shut down.

So, I couldn't select any windows or task bar items, but in the first case CTRL-ALT-DEL worked and in the second case I could still use the mouse to click and select on the Start Menu.

Any ideas why this is happening? A search turned up some other posts with similar sounding problems without any solutions. Is this a common problem and is there a fix for it?

System: Intel Dual Core E2200, Intel 945GC chipset, 4GB RAM, GTX 260