"Microsoft Exchange Server Deployment Assistant" is a tool that will guide you to deploy Exchange 2010 in your organization.
This tool includes the below deployment scenarios and can be access online here
Upgrade from Exchange 2003
Upgrade from Exchange 2007
Upgrade from Exchange 2003 & 2007
New installation of Exchange 2010
Using the "Microsoft Exchange Server Deployment Assistant" is very easy, the welcome screen will prompt you to choose your deployment scenario
After that you will be asked a series of questions depending on the scenario you have chosen.
Each question is explained in details and this explanation can be seen by clicking on the arrows next to it
Finally a report with detailed instructions will be generated can be downloaded as PDF