I was in the process of installing Exchange 2010 today on a W2K8 box when I came across the error: 

The start mode for the Net. TCP Port Sharing service must be set to Automatic before Setup can continue

As it turns out the Net. TCP Port Sharing Service is set to a manual start up

The best way to resolve this is to set the Startup type as Automatic.  You can do this in two ways, one by opening the properties of the service and selecting Automatic or by opening up PowerShell (Which you should have installed prior to deploying Exchange) and typing the command Set-Service NetTcpPortSharing –StartupType Automatic.  Walla!  The service is now configured correctly and you may continue with the Exchange 2010 install. 

*Note: This was deploying Exchange 2010 RC1, this may change with the RTM release.