This has baffled me. I work in Windows XP Home edition. No other computer problems except for this. When I am working in an active window, without any indication, the window becomes inactive. For example, as I am typing this message, this window greys out and I find that all the typing I have done has not registered. When I click the mouse, it reactivates the window and I can START typing again. But then without warning, the window will become inactive again - no particular amount of time and no specific keystrokes cause the reaction. I have run several virus programs and have not found a cause. So, I don't know if its a problem with the computer, the keyboard, the mouse, or what.

Any help would be greatly appreciated. Thanks.I would try a new mouse and or keyboard first as these are the least expensive items to troubleshoot...SINCE the window does become active again i suspect one of the 2 input devices.
ALSO check Control Panel/Power Management and set all settings to never.

patio.  I would check all programs running in the background. It also happened to me and it was the software fo the mouse and the keyboard causing the problem !!Are these a standard PS/2 keyboard and mouse or some infrared or USB THINGS? That COULD be your problem right there.