Hmm okay I installed the bitcoin client already and opened it, but I can not find the place for wallet.dat it is all wallet.ccp/wallet.h and such. Sorry I have not been into bitcoin for a while.
depends if windows or mac or gnu linux it is the directory where the blockchain is stored.
something like Application Data/Bitcoin.
you can find out by starting the bitcoin client once. it will create a new wallet. then search the copmuter for wallet.dat and replace it with your backup of your previous wallet.
the new bitcoin clients do a -rescan so your coins should appear if you have the private key / address in that wallet