Old Email as Apple ID

Before Apple ID's came into force, you could register with iTunes and download music using your email address as your account username.

Since then, I got a new Apple ID when they came out, but I still have a few old albums purchased under the old email address. When I click on them to play, the Authorize This Computer box pops up with the old email address already populated in the field linked to those tracks and I just need to enter the password.

However, I cannot remember the original account password, but when I click on 'Forgotten' to get it emailed to me (I still have access to that original email account), it won't accept the email address because it's not an Apple ID or Apple/Mac/Me/iCloud username or address.

Am stumped. Anyone help???

Thanks in advance.