We're currently running continuous automated functional tests against the CI builds of our software. This is done on a variety of Windows based machines (around 10-15 boxes or VMs), to identify version-specific problems.
The pain starts when we want to upgrade let's say the flash plugin deployed on all those machines, we're currently doing it all manually since I was told a automatic solution is usually feasible when you want to deploy it on 20+ machines which are ideally as identical as possible.
Also we need certain programs to have a specific configuration (a simple example would be the default webpage in IE).
Are you aware of any tool that supports managing applications and setting in that kind of heterogeneous environment?
Is putting all your test machines in an isolated Active Directory installation an option?
You could deploy group policies for things such as the default home page in IE, and you can also push out software too - I deploy Adobe Reader, Flash Player etc. in our office of ~200 computers without issue so 15-ish should be dead easy.
For any other software you can deploy MSI files using group policy very easily, and with a bit of scripting magic you can sometimes deploy non-MSI software too (however this is entirely dependant on the installer).
The Microsoft native solution is System Center Configuration Manager (SCCM), which replaces the old MOM server, and in turn replaced SMS. It can do heterogeneous software deployments via agent which should improve your per-VM consistency of configuration. Best of all, it does it better than Group Policy software deployments.
The down-side is that you do have to invest time and money into packaging software for use with it. As I understand it, you need to be able to package everything into MSIs or simple files/reg-entries. Once you've got this under your belt, spinning up entirely new testing environments is a lot easier.
Build out your machines using MDT and redeploy as required. See Microsoft Deployment Toolkit. you can also pair that with SCCM to implement changes
There is also http://wpkg.org/