Gerrard came up with an issue. The tool will not install on a workstation if you have the Exchange 5.5 Admin tool installed on it. (not tested it with any other flavours of Exchange yet. Let me know)
It looks like the tool is checking the HKLMSOFTWAREMicrosoftExchange registry key.
Gerrard renamed the key and the install worked. He then renamed it back :-|