How to remove backup encryption without the password

I have an iPhone 4S and I turned on backup encryption. This worked fine for over a year but when I updated to iOS 8.4 and tried to restore it would not accept the password. I know the password I used is correct but also tried various other ones.

I eventually used the iCloud backup but would like to use the PC backup as that contains passwords etc.

Is there a way I can turn off encryption on the iPhone & iTunes so I can start from scratch again. I don’t want the old backups just want to make new ones.