Problem with DLL registration

[Matthew Plunkett]

I have an installation program that we distribute through a
traditional setup program and also through install from the web.
Previously, the setup transferred files without registering any DLL's.
Earlier this year, we had to add one self-registering DLL (that our
company created in-house).

The installation works fine as a traditional setup, but chokes when
installed from the web. It fails very nastily, requiring a force quit
from the setup and leaving garbage on the user's system.

The error message it fails on is:

Run-time reference file error.
The run-time reference file needed to conduct this installation does
not exist or contains invalid information. Contact the software
vendor at the installation site.

Any ideas for what is going on?

Thanks,
Matt