This solution may or may not work, but it is worth a shot if you know how to get into the registry and if you are using Windows XP (which I assume you are).
The following comes compliments of the guys over at computing.net:
"Special thanks to UserXP at the belchfire forums.
What causes this sort of error is a corrupt/bad/missing logonui.exe file. I know it might be cool to customize windows but you can mess some stuff up if you're not careful.
In my particular case, I was using logonstudio by stardock (it's free!). Here's the fix:
Open regedit and go to key "HOT_KEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon". There you will found the value "UIHost". Change the data there to point to the full path of a valid logonui.exe. (C:\WINDOWS\system32\logonui.exe)
Exit regedit, reboot.
Mine happened to be pointing to logonuiX.exe which was apparently broken. Follow the steps above though and you should be good and don't be afraid of regedit,
just be careful!"