ah ha I think I know what the problem is. Some how you associated .dat files with bitcoin.exe so that when you double click one windows lauches bitcoin and sends it a command to open the file. Bitcoin sees a incoming command from outside the program and checks to see if there is a password so it can know if it should follow the command. Since there is no password in the .conf file it gives you an error.

You should never have to tell bitcoin to open the wallet.dat file it will automatically find the one in the correct location. To restore a backup you just need to copy the backup to the right location while bitcoin is shut down then reopen bitcoin.

You should never need to open the wallet.dat file and if you do open it you won't use bitcoin.exe to do it, you will use a wallet editing tool.

How are you running bitcoin normal? A shortcut? A .bat file? If either of those try double clicking directly on bitcoin.exe. If you still get the error look in the same folder where your wallet.dat file is for a file called bitcoin.conf and if it is there open it in notepad and look for a setting there.

If you don't get the error when clicking directly on bitcoin.exe check the properties of your shortcut or the contents of the .bat file. If you see some thing like C:\...\bitcoin -server remove the -server.

If you try all that and still get the error then I am out of ideas.

That error is not from interacting with the wallet.dat file it comes from trying to run bitcoin with the -server option enabled without having a password set. Running with the server option on and no password would mean anyone who could get past your firewall and router could spend your bitcoins. So the software does not let you do that.