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...