There is a known issue when installing E12. E12 requires MMC 3.0 and .NET Framework version 2. Even with MMC 3.0 installed, installation fails:
"Exchange Server '12' requires Microsoft Management Console 3.0."
There is a workaround for this problem. Open the registry and navigate to the following:
Add a new key:
Works first time with this key. Checkout the Exchange Team blog for more details.