I always find myself looking for this table so I thought I would post it here as well for anybody who is interested...
Windows Server operating system
Active Directory environment
Windows 2000 + SP3
Windows Server 2003
Exchange 5.5 + SP3
Exchange 2000 + SP2
Exchange 2000 + SP3
This table is swiped from the Exchange Server Compatibility with Windows Server Operating Systems document.
Updated 1/22/2009 – Fixed link.