Can't minimize windows

Windows XP PRO

I'm unable to use the minimize button at the upper right of any windows. When clicked, nothing happens.

Maximize and close still work, and the taskbar seems to be functioning normally otherwise. Clicking on the window on the taskbar itself to minimize doesn't work either.

Rebooting fixes the problem temporarily.
I've also run malware scans.
No recent program changes.
Nothing in startup.

Tried the fix found here (#240) [1] http://www.kellys-korner-xp.com/xp_tweaks.htm which also solves the problem temporarily.

I'm stumped!

What happens when you click on the taskbar button?

Nothing, though it changes color/flickers as though it was clicked.

1) What is new or different since the last time everything worked properly (ie, new hw, new sw, virus, error, etc)?

2) Open msconfig and on the General tab choose "selective startup" (uncheck all three items) and reboot. Does the problem still occur? If not, start adding items back to msconfig one or two at a time, rebooting after each change, until the problem reappears and you'll have identified the offending process. This is clearly a time consuming procedure, but it is the best way to determine if some process loading with the system is the cause of your problem.
After you've isolated the cause, do not use msconfig to permanently disable the process. Instead, if it is a service go to START - RUN and type: services.msc (then press enter) and disable the service OR, if it a program, you can download & run a simple app such as Mike Lin's Startup Control Panel (http://www.mlin.net/StartupCPL.shtml) to enable, disable, or otherwise manage startup programs.