You can simply rename the wallet.dat on your current Bitcoin installation, and temporarily put a copy of this old backup wallet.dat there. When you launch bitcoin with the -rescan option, it will update the wallet to show the correct balance. You can then also send the wallet balance to a new address, if you never expect anyone to send your old wallet payments and want to discard the backup.

Then put the original Bitcoin wallet.dat back and make another dated backup while you are thinking about it!