Kevin,
Could always be wrong on this, but I do not think it is possible to install the full version of 2005 and then run AC. You can install it and even give it a name instance. You can even install AC and install Express, and it will use the full 2005 instance, but it can't connect. I believe the installer is meant to find the express version. Again, not a SQL Expert.
What you can do is upgrade from 2005 Express to 2008 Express or SQL Server 2008 or Advanced. I don't think upgrading from 2005 Express to 2005 Workstation or whatever full version is supported by Microsoft.
I found it relatively easy after installing AC to upgrade to 2008 Express. Of course, while 2008 Express has many advantages over 2005, it still has the same size memory and database size limitations. So, you may want to go with the full 2008 version. The upgrade, which I did on a non-production environment and is running error-free now for weeks, is relatively simple. After the upgrade, SQL 2005 isn't even needed, although sometimes people run them side by side. You pretty much follow the install/upgrade steps and upgrade the AC instance. If you upgrade to 2008 Advanced or any of the full versions, you would be pretty much limitless.
But, as I said, I did this on a virtual machine. I was just playing around. Afterward, I researched it on Microsoft's and other sites, and they walk you step by step through it and give you tips on it. It's funny, because if you just do it, it's simple and not scary at all, but when you read the little nuances, it becomes a bit more eye opening.
Anyway, what I think is the bottom line, is you can't install directly to 2005, but you can upgrade to 2008. I don't know a lot about SQL installs, but I think AC would have to have an option during the install allowing you to choose between Express and allowing itself to make its own instance on another SQL Engine. Luckily, I have a few experts I can run these things by. But, for me, I still have 2005 Express running on my production server and, unless I needed to run many more clients, I'll probably stay there for now. But, who knows? I never seem to leave well enough alone. I generally have to break a perfectly running system.
