I know this is an old posting, but I also accidentally removed the ISAPI-DLL handler, and I found absolutely no solution searching the Internet. To save others from the same futile search, I did find this documented in the IIS Resource Kit.
The handlers are defined in:
C:\System32\inetsrv\config\applicationHost.config.
Scroll down to <handlers accessPolicy="Read, Execute, Script"> section. Therein add the following entry:
<add name="ISAPI-dll" path="*.dll" verb="*" modules="IsapiModule" resourceType="File" requireAccess="Execute" allowPathInfo="true" />
Order probably doesn't matter, but after ASPClassic is where the missing handler (if you deleted it as I did) originally resided.