Sounds like this "old" wallet.dat is either:
- corrupted, so that the encrypted private keys are unable to be decrypted properly. You might be able to use the "wallet-tool" (look in same directory as bitcoin-qt and/or bitcoind) and use the
salvage command. It
might be able to recover some of your wallet. If it doesn't work, there likely isn't really much else that can be done with this wallet.dat unless you happen to have other backup copies you can use
or
- it is one of those fake wallet.dat's that have been altered to appear to contain hundreds of BTC, and are then sold for like 0.1 BTC claiming you could get 10's/100's of BTC for cracking the password... however the encrypted data to the keys with the 10s/100s of BTC is "fake"/junk data so it is unable to be decrypted.
Did you buy this wallet? or is it legitimately your original wallet?
If you bought this wallet.dat or downloaded it from some website, please be honest, otherwise you will be wasting everyone's time trying to help with a "fake" wallet.