UAC appears to prevent logon from primary account

I recently purchased a new laptop for a contract. Since I could no longer
purchase XP, I upgraded to Vista Ultimate. I set up as much of the software
as I could from the local PC account. At the client site, we set up a domain
account on the client network which I used to set up the remainder of the
development environment. The project uses some Com dlls written in VB6,
Classic ASP pages as well as ASP.Net.

The first real problem was registered dlls that were not available to the
IIS process. We used the SysInternals process monitor to determine that the
process could not read the HKEY_CLASS_ROOT table in the registry. As a work
around we added the IIS user to the users group then gave the users group
access to the registry for the dlls that were causing the problem. After
considerable frustration, I finally got all the bugs worked out, or so I
thought.

The next time I went back and tried to log onto the original PC account, the
screen went black after the welcome screen. The domain account continues to
work OK. I am able to go in in safe mode and after experimenting I discovered
that I can logon if I turn UAC off. Apparently there is something installed
to which the login process for the second account does not have access.

I have tried using process monitor from the second account while logging in,
but that has not helped. How can I find out what is causing the problem?