I have this same problem when i have MultiBit wallet. I make backup of wallet.dat file and later i have question what i have to do to bring my wallet back.
With MultiBit it is easy - you have to install Multibit Classic, then just copy -> paste your old wallet.dat to %appdata% folder with Multibit, and wait for actualization.
Multibit does not use wallet.dat files. Attempting to use a wallet.dat file with Multibit will not work.
my bad - i write xx.dat - but i'm sure that you know that i was thinkin about xxx.wallet - and it is very easy to safe data for newbies..
Edit: sorry, now i get the difference between .dat and .wallet - sorry for confusion