rundll32.exe

Basically it is a program that allows you to use and extract data from dlls. Microsoft designed this as an executable that would run dlls and then extract data from the functions within the dll. Check this site out for more info.

http://support.microsoft.com/default.aspx?scid=kb;EN-US;q164787

be careful as this is where the nimda virus starts saying u dont have this file
go to norton and download the nimda fix and run it see if this is what is making this pop up