Make sure that you are copying the backup to the right place. Right click the wallet.dat file in the data directory and click on properties. Make sure that the "Created" field has a date that makes sense for that wallet to have been created (e.g. 2013).
If you have made any transactions between when that backup was made and when you imported it to Bitcoin Core, it is possible that the backup does not contain the private keys that control your Bitcoin. Since Bitcoin Core is not a deterministic wallet yet, you need to have made a backup of the wallet every 100 transactions.
Do you see any transaction history?
Find an address that you know should have been on that wallet. Then go to Help > Debug Window and click the Console tab. Then in the box at the bottom, type
where <address> is the address that you know should be on that wallet. This will give you the private key for that address. DO NOT SHARE IT WITH ANYONE. If you do get the private key, it should be in black text and begin with a "5", "K", or "L". If you get an error in red text, then that means the address is not in the wallet and you have the wrong wallet.
If you do have the wrong wallet, try copying the backup in again.
You should also try getting the wallet from the windows drive. Make a live CD or USB of another OS such as Ubuntu. Then boot from that on the windows pc and try to access the Windows hard drive. Then pull the wallet.dat file from there.