Yet another Keyboard (non-USB) error at start-up

I looked through about 85 pages of posts about this, and didn't really find one that seemed to fit my exact issue.

The basics: Dimension 4500 (tag 4FGSN11), Win XP, old IBM-brand standard PS/2 style keyboard

• Have had problem since beginning (July 2002), just never got around to tackling it
• 90% of time, system boots up fine and no problems
• 10% of time:
• • get "Keyboard error" message and beep during POST
  • system continues to boot normally
  • mouse (non-USB Dell Intellimouse) works fine
  • as soon as I touch a key on the keyboard, system freezes completely; no mouse, no keybd, no nothing (except frozen Windows XP desktop)
  • to remedy
  • • must dicsonnect power to shut down
    • reboot; same keybd error occurs
    • using mouse only, execute XP System Restore
    • XP reboots and all works fine
• Symptoms \ coincidences \ observations
• • when the problem occurs, I go into System Information > Devices >  (using mouse only) and notice that there is no keyboard device driver listed at all (presumably, this is what XP's System Restore re-installs to get everything normal again.
  • the issue seems to occur *only* when I have either added or removed a USB device sometime prior to rebooting the computer
  • I have tried using different USB ports for my devices; same results; also tried using a self-powered hub.  No joy.
  • It is not keyboard specific.  I've been using the keyboard in question on other systems for years.  The keyboard also currently works on an older Dell Dimension I have.  I've also swapped out and tried other IBM keyboards (I have 6) with the same problems. 
  • I have not tried using the Dell Quietkey keyboard, and am not even sure I could find the one that came with my system (can't stand them; gotta have the heavy tactile response generated only by the old-style IBM)

Any ideas?

Thanks.