On the old computer, the file you are looking for is 'current.gpx'. I have forgotton where it is stored on an xp computer and am too lazy to start my old laptop to find out. Search for it.
On the Win 7, the file will be at C:\Users\<your name>\AppData\Roaming\GARMIN\GarminMobilePC\gpx. Copy the old file and replace the on at that location in Win 7 with the one you copied. It will have all the information you had before including routes, tracks and POI.