and what might that error message be> we only know what you tell us.

Here is the message that keeps coming up:

Error with account "my e-mail address" The account "my e-mail address" is being processed. Please wait until processing is complete to get messages.

Well the most common cause of that is anti virus scanning your profile folder and existing mail. We recommend creating exceptions in the anti virus program for you mail and imap mail folders in your profile. https://support.mozilla.org/en-US/kb/profiles-tb#w_where-is-my-profile-stored

The other cause is a really slow computer that checks mail often so the indexing service never actually gets done before the next mail retrieval. IS you computer slow? Given it is XP I guess it could be very old an slow.