You want to move the family to a new Linux Distro - Mint and Ubuntu are very nice, easy to install, and have a nice array of included/installed software. You can use dual boot with an old XP partition and show them how to load their old files.
If they want to just surf, watch videos, type papers, etc. then Ubuntu/Mint is a great choice.
If they want to play games on the PC, then Windows 7 is the best choice.
If they want a terrible headache-PITA, then OSX86 may be the ideal choice (it's not, lol).
If they want to add/subtract/multiply/divide, then a TI-36 Calculator is probably for them.
Give them the choice, and show them the +/- of each. Although, if you show them Compiz effects they'll be hooked like I was some 5 years ago.