That's the problem with having multiple itunes accounts - you will forever need to remember the login credentials for all of those accounts.
When you buy a song from itunes, the account ID/user name is embedded into each file. Just right-click on a purchased item and Get Info - it's there on the summary tab.
See if this page helps, with the "iForgot" tool, if you don't remember the password
http://www.apple.com/support/itunes/store/password/
Did you *sign out* between each account and then authorize the PC?