David and any others interested, I believe the InvalidCastException when 
calling a remote COM component from ASP.Net was due to security 
permissions.  I made the following changes:

1-I added <identity impersonate="true" /> to my web app's web.config.
2-In IIS, I made my app a separate app by setting the Application 
Protection setting to High (Isolated).
3-In Component Services, I set the Account Identity for this app to a 
specific domain/user.

Tim