You are wanting to access the private guid key within the registry but see more than one. To locate the correct Guid, within Exchange server 2003 follow the below instructions:

1) Launch Exchange Server 2003
2) Expand Administrative Groups
3) Expand Server
4) If you have more than one Exchange server, expand the one as required
5) Expand first storage group
6) Expand Mailbox
7) Click the folder named logons and on the right hand pane you should be able to locate the GUID

The GUID for public folders is different but you should only see one registry folder for the public store. Incase there is more then one, follow the above instructions but select public folder store.