You must be a manager. I'm not trying to be rude, but you are either not telling us everything or clearly do not understand Win2k/E2k.
1) Are you in mixed mode?
If yes, then this makes a little sense, because the user could have a logon password, and the mail servers could be in a different domain without a trust, so they would need another id and password to access their mailboxes.
However, reading this thread it sounds like you are in native mode, which is even more confusing. From the ADUC (Active Directory Users and Computers) you should find the person, and get their properties. This is important. Do you see Exchange attributes on the object (user account). When you install Exchange 2k in Win2k you will run forest prep and domain prep (or just install and it will do it for you) and that extends that Active Directory Scheme to add Exchange attributes to the directory. So before you install E2k, if you looked at a user in AD you would see nothing about Exchange. After you install Exchange, you will see Exchange attributes.
Anyway, you might have migrated some accounts, and or, when you created an account you did not email enable it. You have to email enable an object so that it can have a mailbox resource associated with it. Have you done this? If so, then the users account password will be the same password that he/she uses to get into his/her email.
If the person is mail enabled, and cannot log into email, then their might be a problem with AD Replication. But one year? That's indicative of a severe problem in your environment or the users client is not installed correctly (i.e. internet version of O2K vs. Exchange Server Corporate).
Open ADUC, right click on the person and you should see "Exchange Tasks". From here you can enable instant messaging, move mailbox, or delete mailbox. If you want to get their mail out, then use ExMerge to merge it out to a .PST, then blow their mailbox away and recreate it. I think you have bigger issues, nonetheless, I think I have covered everything I can think of.
Juan Delegator