Rundll32.exe is a process that runs DLLs and stores their libraries for more efficient use by applications. This program is important for the stable and secure operation of your computer and should not be stopped. Check if the rundll.exe file is in the System32 folder. If this is the case, it is Microsoft’s legitimate process, otherwise it could be a virus.
The following is mainly based on the fruitful discussion at TechNet, and the important points have been selected from there and presented here.