Following up on this thread: Windows XP outshines Vista in benchmarking test, the software used is DMS Clarity Studio - a free testing and performance tracking suite. Basically, the test runs a rapid succession of tasks in MS Office and IE and reports the completion time.

Register here, then once you login to the site you can download the Studio (and Tracker if you want to make your results available online). Then, after you launch the program, select the "Framework" panel and select "Display Results" - the options above that apply to the optional tracking and upload features.

I had the fortune of communicating with someone from DMS, and he recommended the following testing procedure:
1. Boot the OS and allow 5 minutes for the system to "settle" (background services to complete loading, etc.)
2. Run a single pass of the benchmark to allow for caching of the application code, etc.
3. Run a 10 iteration loop of the benchmark and take the average score across those iterations.

These are my results on Vista Business x64, XP Professional, and Server 2008 Enterprise:



On a side note, I found that it might take Vista 10-15 minutes to become completely idle, so you can expect worse times if you try to run the test while the hard drive is tied up

No takers? It's a pretty neat visual experience, if I may say so.