Only Wallet version 1.2.5:
If either you are restoring the wallet into a new computer with a brand new installation, or you are restoring an old backup, please follow these steps:
1) Please click menu "File" in QT wallet and click "Import Wallet"
2) Locate you wallet.dat file that you sould have backlup previously.
3) Click "Open".
The QT wallet client will load the wallet.dat you specified and will restart the wallet and start to resync. Let is finish and do not close it until fully sync.