First try to restart your computer. I some cases it might be a sufficient remedy that will let the Windows fix the problem by itself. As an example, if the error had occur after the improperly plugged off USB device. Because some causes of the problem might be related to incorrect and outdated registry records, try to run some advanced registry cleaning program that will fix this kind of possible causes for the error.If you noticed that the problem occurs after you had installed some new software, then try to uninstall it and check if the error will go away. The problem also might be caused by some faulty device driver, and so, it is better to make the system update before you start to make any changes.
If you already updated your system ran the antivirus and searched for possible files with wrong name aliases, and deleted them, then you need to replace the corrupted rundll32.exe in original location with the legal copy of this file. It isn’t that easy because Windows system is protected from the system program files replacement, especially Windows 7 and Vista. Even if you get a right copy of rundll32.exe from another PC or download it from the Internet, might be not able to replace the one in the folder System32.