There is no reason I know of that GSAK should interfere with Mapsource to the point it won't run. I'd be tempted to contact Garmin or search their suport section. Possibly the registry settings are buggy and it might have to be fully uninstalled in order to work. I know there have been some problems with Mapsource and Vista.

You probably don't NEED Mapsource (I very rarely use it), but it's a nice tool to have when you do need it, especially when loading the Garmin Maps.