If lfs is compatible with your parents, it will just unlock. If it isn't, the main hard drive will be reformatted.
Quote from Anman :What will happen if i try to unlock LFS on my parents computer? Do you mean what happens if you have 2 LFS Games and 1 Licensed name on both Games?
Yes. Quote :You may install S1 or S2 on 2 computers for your own use or to race a friend on a Local Network.