You can extract a new copy of rundll32.exe from the Win Me cab set.

Rundll32.exe is contained in WIN_17.CAB of the Win Me cab set and can be
restored using MSConfig | General | Extract File ... and following the
prompts.   The "cab" files are in the Win9x folder on the Win Me CD and
also in either C:\Windows\Options\Cabs (if an OEM install) or
C:Windows\Options\Install (if installed using a Microsoft Win Me CD).  See
MS KB129605 - "HOW TO: Extract Original Compressed Windows Files"
(http://support.microsoft.com?kbid=129605).

Rundll32.exe belongs in the C:\Windows folder

Alternatively use system restore to roll your system back to before you
deleted rundll32.exe provided that that was not more than a few days ago.