Exchange Server 2007 is more than an e-mail system. Exchange Server 2007 has several functions, from e-mail messaging, calendaring and real time collaboration to Unified Messaging. All these Exchange components must work together and several components from Exchange also rely on a lot of Windows Server components like DNS, Active Directory and IIS.
This article from MSExchange.com will show you how the new Exchange Server 2007 core services interact with other Exchange and related Windows services and which services from Exchange Server 2003 are obsolete in Exchange Server 2007.(continue at source)