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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:
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.