Just a note in case anyone runs into a similar problem.
I installed SP3 from the 324MB download file on an XP Pro SP2 system
with two users defined (one normal and one admin). After going
through the recommended restart I tried to logon as the normal user.
The system presented a popup stating that the user's profile could not
be found. Sure enough, none of the user's customization (desktop,
start up, etc) were there.
As part of the online chat with MS support (using the admin account),
I pressed ctl-alt-del twice and only the admin user appeared. However,
when I switched to the normal user, all was back to normal. Going back
to the admin account, ctl-alt-del twice showed both users.
I rebooted again just to make sure everything was really OK. Somehow,
using ctl-alt-del twice appeared to fix whatever the problem was.