Here's an interesting problem that I just came across. One of my customer's XP computers would boot normally, except the system tray was completely blank. No icons -- just a blank bar where you wold normally see the system icons.
I checked to make sure the system tray hadn't been move to the side or the top of the screen.
I eventually found this note on one of the forums that solved the problem.
Basically you press CTRL-ALT-DELETE to bring up the task manager. From that application you can run REGEDIT from the File > New Task (RUN) option.
Once in REGEDIT you delete the following keys:
Reboot and the system tray should be accessible. In my case the system tray was squished down to just a sliver. However, I was able to move the cursor over the sliver and resize it so it looked normal.
Weird, but it worked.