Rundll32.exe is a process which executes DLL’s and places their libraries into the memory, so that they can be used more efficiently by applications. This program is important for the stable and secure running of your computer and should not be terminated. Check and ensure that the particular rundll.exe is situated in the System32 folder. If so, it is the legitimate Microsoft process, else it could be a virus.
The following is, primarily, based on the fruitful discussion at TechNet, and the important points have been culled out from there and presented here.