Error installing ActiveX cab on Windows Vista

I have a 3rd party ActiveX control I distribute with my software that fails
to install on Windows Vista. When I install it from the browser, it prompts
me to install, appears to have been successful, but the install fails
silently. Using the MS ActiveX log utility, I can see it fails with an
error: "Code Download Error: (hr = 80004005) Unspecified error".

If I manually copy this ActiveX to the target system and run regsvr32 on the
cab file, it gives me an error telling me the control "may not be compatible
with the version of Windows that you're running." If I extract the unsigned
OCX (the cab is signed; OCX is not) and run regsvr32.exe on this OCX, it
installs and runs fine.

This cab is distributed by a vendor and works on all other platforms. Oddly
enough, it only fails on SOME Vista systems.

I tried all the usual Vista tricks (added to secure zone, lowered security
in secure zone, changed Advanced IE settings related to certificates, tried
the ActiveX installer service, and disabled User Account Control).

Anyone have any pointers?

Frank