Program icons not responding

Windows 2000 advance server
If I leave the computer idle for about 2 hours, the following happens:
- icons on the desktop do not respond to mouse clicking: the sand clock shows up for less than a second, disappears and nothing happens.
- icons onthe left side of the task bar, adjuscent to "Start" begave the same way.
- active apps i.e. icons adjuscent to the time logo respond correctly to the mouse click.
- ATI launch pad:TV, DVD, Video CD, CD Audio and File Player work fine.

What I doas temporary solution: Control Alt Del, log off, log in again, and all in good.

Does anyone know what the problem is?
My computer is a compaq with AMD 700, 256MB or ram.

Thanks in advance for your valuable response.
Cyrus