My VB.NET app creates and populates a worksheet in Excel each time that it
does it's thing. Every time the user selects a doctor from the list and hits
the OK button, I want another sheet to be created and filled.

So now what if the user closes Excel? At present I am simply reusing the
Excel object, such that if they close Excel my program throws an exception. I
want to be able to trap the exception and either startup a new instance of
Excel or attach to an already running instance of Excel.

Also, how do you trap exceptions for Excel COM calls?