Only downloads from iTunes will be tied to that iTunes account, unless you subscribed to iTunes Match and uploaded the other songs from your computer to it. If you can't remember the account's password then you could try seeing if you can get reset via this page : http://iforgot.apple.com
If that doesn't work then you could try contacting iTunes Support and see if they can help : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page
When you've got access to the account you can update the primary email address on it to be one that you still have access to : http://appleid.apple.com
If you don't have the music that wasn't downloaded from iTunes on a backup then there are some programs listed half-way down this page which should be able to copy them back from your iPod : https://discussions.apple.com/docs/DOC-3991