Why bother with a demo?

Buy the game, try it and if it doesn't work get a Steam refund (playtime must be less than two hours).

http://store.steampowered.com/steam_refunds/

...if you buy a digital copy through Steam, of course. I kind of doubt they'd do a refund on a game you bought on disc in a store (or got in your Pip-Boy edition), since they didn't get the money from you for it.