This is likely a profile issue. Try this. Create a new user and try switching to that user.
Be aware that SP2 upped the security levels and given that multiple logins can present new security issues, troubleshooting this will be fun. If I was to dive into it, I'd have to be there to observe every nuance. That won't be happening and it's up to you to try switching between new users. Then between one old user and a new user, then the other user and hopefully you can sort out what each profile is doing to trip it up.
In closing, you will also learn how to use the EVENT VIEWER which is noted in Start, Help and Support to dig for clues.
Best of luck,
I upgraded to SP2 shortly after it was released. The download and install went smoothly. However since the upgrade I am unable to switch users on my system without logging off on one user and then back on to the next. When tring to use the switch users feature, I get the users selection screen and when selecting the new user the screen flashes like it is going to switch then returns to the user selection screen.
My system has 768MB memory, an Athlon XP 2800 processor, 40G disk with about 8G free. There are 3 users set up one for my self and each of my kids.
Does anybody else have any ideas?