The best idea would be to back up for vghd folder, which has underneath it two sub folders that contain all the program (both data & model) information.
Now its location will vary depending on what operating system you have:
Windows XP: - C:\Documents and Settings\your username\VGHD
Windows Vista\7: - C:\Users\your username\AppData\Local\VGHD
I am not 100% on these as I do not install the data to the default location and have not done so for a very long time.
Its my opinion that when you install, you should change the data to a drive that has a lot of spare disk space, preferably a separate HDD if you have one.