In that case, you are probably being prevented from editing that file by one of Vista's security features. Vista does things very differently from XP when it comes to what's installed in Program Files and how it is handled. There are situations where even being an Admin isn't enough to let you change files that are in Program Files.
There are a couple of things we can try. The first is looking for a 'shadow' copy of that file and deleting it. Vista, in certain circumstances, puts a backup copy of changed files in a special directory called VirtualStore. You'll find that directory here: C:\Users\<your user name>\AppData\Local\VirtualStore\ (you'll need to select your user account where it says <your user name> when navigating to that location). If you find anything in there related to Virtual Families, you should delete it, and again try editing the file I mentioned earlier.
If that doesn't work, you should completely uninstall Virtual Families again, reinstall the game, only this time
do not let the game run immediately after installing it. Uncheck both of those check boxes, and close the installer. Then run the game and see if it will launch properly (in full screen). Leave it in full screen, quit the game and try to launch it again to see if it still works.
And, please, let me know what happens.