Your old Apple ID, someone else's, or and old email address that you've now changed on your current Apple ID?
When does this ID pop up, e.g. when you try to sync or play certain content?
Either authorise the computer for the account if it is yours (i do this), or remove the content from the library.
If you've changed the email address associated with your Apple ID you may need to delete old copies of protected content and redownload. (How do i do that? you use to could update the apps and the ones that would not update were the ones with an old ID)
thanks for the post