|Go to "C:\Windows\System32" and make sure rundll32.exe is really missing.|
If so go to "C:\Windows\ServicePackFiles\i386" and copy the one over from there.
Another approach is to type sfc /scannow in the run box. If it runs it should restore missing system files.
If you are unsure about whether rundll32.exe is really missing then try the EXE fix on here (which is the usual fix for program file associations):
Always pop back and let us know the outcome - thanks