When uninstalling Exchange 2000, before you start the uninstall you need to verify if the exchange home server attribute is set to the server your trying to remove for any of the users. You can check this by searching with active directory users and computers at the domain root level. Search for show only exchange recipients on the exchange tab. Then go to edit --> choose colums and add in the Exchange Home Server colum.
In the screenshot below there is a user that has his home exchange server set to PDC, the 2000 server we are trying to remove. We can either use adsiedit to point him at another server, or delete the account.
To remove it using ADSI follow this method:
1. In the ADSI Edit MMC, expand Domain NC [domaincontroller.example.com], expand DC=example,DC=com, and then expand the container in which the user who has the mailbox server listed in the msExchHomeServerName attribute is located. For example, expand CN=Users.
2. Right-click CN=username, and then click Properties.
3. In CN=username Properties dialog box, click Both in the Select which properties to view list, and then click msExchangeHomeServerName in the Select a property to view list.
4. Click Clear. The LDAP path of the mailbox server appears in the Edit Attribute box.
5. Remove the contents of the Edit Attribute box to make it blank, press SPACEBAR to let the Set button become available, click Set, and then click OK.
6. Follow steps 2c through 2f to modify the msExchHomeServerName attribute for other user accounts that have this mailbox server set. Then, exit the ADSI Edit MMC snap-in.
See Microsoft Knowledge Base 924170 for more information on this.
If you dont do this you will not be able to uninstall Exchange 2000 and you will recieve the following error:
Additionally make sure that you have the Exchange 2000 disk as this is needed to uninstall. This is because during the installation some dll's are replaced with exchange enabled ones. When you uninstall it needs to restore the originals.