With the release of Windows Server 2008 coming officially in February of 2008, it seems pertinent to begin application testing with Windows Vista Service Pack 1. (Note: Windows Vista SP1 final bits will most likely release with WS2008 release).
The first step is to install Vista SP1.
Here are some useful links:
Windows Vista Service Pack 1 (SP1) Release Candidate
http://technet.microsoft.com/en-us/windowsvista/bb738089.aspx
Microsoft Application Compatibility Toolkit 5.0
http://technet.microsoft.com/en-us/windowsvista/aa905102.aspx
Happy New Year 