The subject line basically says it all.
For some reason, the computer locks hard when I do a user switch,
whether it's a "fast" user switch, or a "logoff" user switch (there
are three accounts on the machine).
The PC is updated as far as I know, and has been checked thoroughly
for viruses and spyware.
I cleaned out all temp files, too and disabled power management for
the machine, as well as the onboard NIC card.
Anyone else have ideas?

I am not an expert. I do have a XP Service Pack 2(home edition) I got
it about a month ago. With it came a PC Troubleshooting and
Maintenance Guide (hard copy only).
Under Hard Disk Drive:
Symptom - PC seems to be locked up and not responding.
Possible solution - Use the Windows Task Manager to close any programs
not responding or to restart the PC:
1. Press the Ctrl key, the Alt key, and the Delete key on the keyboard
at the  same time.
2. Select the program that is not responding, and click END TASK.
   - or -
   click SHUT DOWN, and then click RESTART.
If this does not work, press and hold the On button for 5 or more
seconds to turn off the PC. Then, press the On button.

Under Performance:
Symptom - Applications and files take longer to open or respond.
Possible solution - If you created multiple user accounts on your PC,
make sure other users are not logged in. If there are multiple users
logged in at one time, system resources must be shared amoung the
multiple users.

3 other possible solutions are
1. Disk Cleanup program
2. Disk Defragmenter program
3. Remove unused programs

Also, on the PC, under the Main Help Menu I have the option to contact
a Technician. Telephone numbers are supplied for numerous countries.
System Information is on the same page.

Hope this helps.

I had similar problems when McAfee anti virus was running.

Try disabling it if that's your AV software - and go with AVG from grisoft instead.

See this post

http://www.programwitch.com/main.asp?xP=BlogLink&xM=1&xY=1584

Perhaps XP's log of events might provide a clue. Microsoft has a page
that can walk you through opening and using the Event Viewer:
http://support.microsoft.com/?scid=308427

Usually while switching users pc will get locked, then u needto login
in different user