Did you have maps installed in Mapsource on your old computer? Did you copy them to the new computer? If so, getting Mapsource to run is just part of the battle. Simply copying the files from the old computer will not make the maps accessible in Mapsource.
This is due to the fact that Mapsource depends on Windows Registry entries to find its maps, and simply copying the files does not create the needed registry keys. This is where the GMTK program comes in handy, it creates the registry keys when you use it to transfer maps.
I always thought using the Windows registry to access maps was a really bad idea, but maybe it made sense back when Mapsource was first introduced? Garmin created a new map format called "gmap" perhaps 10 years ago when they began supporting the Macintosh. The registry approach doesn't work on the Mac, since there isn't any registry. Garmin is now using gmap on both Macs and PCs, and these files can simply be copied from one place to another without worrying about registry keys.
There are still plenty of the old registry based maps around though such as old Garmin products, and third party mapmakers have been slow to embrace the gmap format.