This was one of two issues we had had right off with MediaLab on Vista. Try downloading and reinstalling the most current release of MediaLab (v2008.1.33, release April 6, 2009). We attempted to address this known issue and had success in many cases but not all. It can be found on our main download page here:

www.empirisoft.com/download.aspx

Out of curiosity, are you already running this release, and if so, do you know approximately when you downloaded/installed it?