James, thanks for diagnosing as you have. 
As much as Dale is normally spot on, it does seem that something in Vista SR2 might have caused Microstudio 4.20 to malfunction.
You are way beyond the "top end boundary" of number of files to import" into the My Songs list. Microstudio was not designed for this type of "mega-use".
Having said that, we will address 4.21 Beta 1 running on Vista SR2. 
Please post what "Vista version" you are running (Home, Premium,etc.) so we know exactly what you are running.