Windows XP lockup at logon screen

When I leave my computer and it times out, it goes back to the logon
screen. Fine.

About 1/3 of the time, when I try to logon, the window won't let me.
The mouse pointer moves, but I can't get any of the usernames to give
me a box to type the password in. The only way out is to shut down and
reboot.

Is this a common problem? What is the solution?

(I am using XP Professional with SP2 build 2600)