HKLM\SOFTWARE\Microsoft\Microsoft Integration\{GUID}\PID3.0
This was the case on a Windows Server 2000 install with Exchange Server 2003 installed, when I get the chance I will test this on Server 2003.
Retrieving Your Exchange Server 2003 License Key
Written by
Sam McGeown
Sam McGeownThis article is now 15 years old! It is highly likely that this information is out of date and the author will have completely forgotten about it. Please take care when following any guidance to ensure you have up-to-date recommendations.
Recently I found the need to retrieve the key from an existing Exchange Server for a reinstall - the software is legally licensed but the key was somehow lost. A trawl through my registry revealed that the key is stored in an obscure place: