How to use Backup .dat? I had a Bitcoin wallet with come coins, backed it up (have the .dat file and a .csv file) but recently had to reinstall my Operating system which sadly wiped all my programs.  Now I have a new wallet but do not know how to access my coins using the .dat...  any hints?
Find where you wallet file is located which will be different depending on which operating system your using. Then replace the new wallat.dat file with your old one. If you haven't downloaded the block chain yet, then just start up Bitcoin-qt and once it downloads your coins will be in your wallet. Otherwise you will need to re-scan the blockchain at which point your wallet will have your bitcoins in it.