You can do 2 things.
Export all in GTC at the old computer by right click on a map and select export.
However if you have 2 sub maps with the same name, although they have a different parent, GTC export always the first occurence of the directory of that name. If you don't have same names, this will work.
Or go to the right directory in the file system, on a XP machine this is C:\Documents and Settings\All Users\Application Data\GARMIN\Training Center\TCAcct.
In this directory there are files like HistoryTree.bin, CourseTree.bin, UserData.bin, UserProfile.bin, WorkoutTree.bin.
Copy these files to the TCAcct directory after installation of GTC on the new PC
1. Install GTC,
2. Stop GTC,
3. copy the files,
4. Start GTC and it should recognize the user data.
I haven't done this myself. But this is the solution provided by Garmin