BleepingComputer.com: Corrupted Windows system graphics

I usually 'switch user' to log off when I'm not using the computer.
After several hours, when I try to log back in, something has failed, and I get the official 'Log On to Windows' popup instead of the full screen with my name beside a password entry box.
After I log on, system graphics are corrupted, and it seems Outlook Express is especially non- functional.
'Start Menu' is missing various graphics, the Start button itself may have black letting instead of white, and when I bring up the shutdown popup, it's usually clear with the buttons showing for standby, shutdown or reboot. Sometimes I'll get a system error popup.
In any case, a reboot always fixes the problem.
My system disk has C: for Windows and D: for My Documents
I have, in approx order:
Cleaned out all spyware infections, etc
Tried my 6 mo old copy of my disk [actual physical disk]
Reinstalled WinXp [Professiona] over existing installation
Replaced disk
Reformatted C , reinstalled WinXp Pro clean, reinstalled about half of my apps [keeping D:]
Tried different graphics card
New motherboard
New memory
Reinstalled old cpu
Haven't changed system power supply
It's a ATX mid tower with the side removed
Photo of recent failure showing Outlook Express, Start menu, and popup:
http://www.flickr.com/photos/pixelpushing/3667914191/