How to create a new "password-free" backup to my iphone

Hello,

I have an old iphone backup in my PC that was encrypted with password.

I don't remember the password so I want to create a new backup without a password.

I deleted the old backup from the PC (using itunes preferences) and now the itunes reports "Your iPhone has never been backed up to this computer"

When I tried to remove the marking from the checkbox "Encrypt iPhone back" , in order to crearte a new backup without a password, the itune asks for the old password (the i don't remember) and why? there is no backup file in my computer.

Now I'm really stuck - I want to create a new backup and I can't because I don't remember the password that was in use in the old backup file that I deleted.

What can I do to get out from this dead-lock ?