You would have to modify the GarminDevice.xml file to point to the new location. If you have XP that file would be located at C:\Documents and Settings\<your name>\Aplication Data\GARMIN\Devices\Garmin Mobile PC.
Make a backup copy of the GarminDevice.xml file with a different name (e.g. Garmin Mobile.bak) and open the original xml file for editing by right clicking on it and choosing 'Edit". Look for references to <path> within the <path> section of the file. Change each of them to point to your new location.There will be more than one with either a <basename> line or <FileExtension> line directly below it.
After you have changed the references, save the file again and see if Mobile PC will open and find your new map location.
NOTE: I have not tried this myself and if you accidentally screw up this xml file with a software-only version of Mobile PC, you will lose your activation!!
The easiest way to make Mobile PC put the maps where you want it to is to tell it to do that when you install it. However if it is already installed, you can't tell it to change by reinstalling.
EDIT:
Another safer way you might try is moving the files where you want and putting a shortcut for each img file (i.e.GMAPPROM.img,GMAPBMAP.img,GMAPSUPP.img) in the original GarminMobilePC folder. The large one will be the GMAPPROM.img file.
Terry