If it's not caused by spyware, here is a neat way to track down what the Rundll32.exe is executing.
Click Start, Run and type CMD
Type the command given below exactly:
WMIC /OUTPUT:C:\ProcessList.txt PROCESS get Caption,Commandline,Processid
(Works in XP Pro)
Now, open the file C:\ProcessList.txt. You can see the details of Rundll32.exe process in that file.