You still need to find the GUID, you can do this by going to ADSIEdit.msc.
Expand Configuration, Services, Microsoft Exchange, Organization Name, Administration Groups, Your Admin Group Name, Servers, Your Server, Your Storage Group.
Right click the Message Store in that group and look for OjectGUID. This is written backwards from the Registry key.
Example
Private-12345678 in the registry Will be written as 0x78 0x56 0x34 0x12 in the ADSIEdit ObjectGUID
You can use the following article to match the objectGUID to the proper registry key:
http://support.microsoft.com/kb/899663/en-us
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For Exchange 2003 you can also use "Exchange System Manager" or "Users and Computers" to find the GUID for a particular database.
If you need to find the GUID of a database so you know the right registry key to go into, use ESM. Navigate to the database in question and display the mailboxes it contains. There will be 2 mailboxes in each database that display the GUID:
SMTP (ServerName-{GUID}
SystemMailbox{GUID}
In reverse, if you want to find which registry key relates to which database, using ESM and browsing through each database to look at the mailboxes can be slow. Open up "Users and Computers" instead, expand the "Microsoft Exchange System Objects" container and you'll find all the SystemMailbox{GUID} user accounts associated with the system mailboxes in your databases. Open the properties on the account that contains the GUID you're interested in, the Mailbox Store is shown on the "Exchange General" tab.