Try your luck in Safe Mode, use Search and see what you find for "rundll32.exe".
You SHOULD have it in the System32 folder, a copy MAY be found in both old or current ServicePackFiles folders. You MAY also have a copy in the DllCache folder, anywhere else would be suspicious.
The version I have in SP2 is 5.1.2600.2180 the size is 33,280 bytes.