Also try to find it on your computer. Usually a copy of it can be found in c:\windows\system32\dllcache (a copy from c:\windows\system32) or c:\windows\i386 or c:\windows\$uninstall that is a folders where updates saves copies. If found then copy to c:\windows\system32. rundll32.dll is essential windows process. If you don't have then simply to re-intall windows is your choice.
P.S. Do not download it from the internet.