There is a customer of mine that is running a simple workgroup in their office, with some xp home and pro machines mixed. They have a machine running XP pro SP2, updated besides SP3. This is kind of the "shared" pc between several users. They use it mostly for IE and MYOB.
In user settings fast switching is enabled, and up until about a week ago it was working great, if a user left the machine without logging off it would simply go back to the welcome screen and another user could select their account and simultaneously log into the machine. Now when the user clicks to login the desktop starts to load for about a second) then flashes back to the welcome screen. If i log back into the first users account and check the task manager (and check show processes for all users) it doesn't show that the second user is logged in. When the user logs out correctly then another user can login fine. This is true no matter who the first user is that logged in. I've disabled fast user switching and rebooted. Then re-enabled and rebooted but nothing seems to allow that second user to log in.