fast user switching

I have a dell dimension 4600 windows xp home version 2002 sp2 Pentium 4 cpu 2.66 ghz 512 mb ram

I've lost the fast user switching function. When I click on change the way users log on or off I get a message that says: A recently installed program has disabled the Welcome screen and fast user switching. To restore these features you must uninstall the program. The following file name might help you identify the program that made the change: MrvGina.dll

So how do i go about trying to find the program that is the problem?

I can find that file, but i can't delete it.

It is a real pain not having this feature once you've been accustomed to using it.