However as most Microsoft files are distributed in CAB format, the file that you're after is probably in one of those.
You can use the EXPAND utility to locate the file. Something like:
EXPAND *.CAB -F:MSO9.DLL c:\temp
whilst in the directory containing the cabs will copy it to your c:\temp directory.
Then move it to the correct place. e.g. c:\windows or c:\windows\system etc.
Bear in mind that the file that you're after might come from Microsoft Office, so make sure you scan those CABs too.