Please see Microsoft's OS guide for Exchange 2010: http://technet.microsoft.com/en-us/library/aa996719.aspx
In brief:
Operating system on a computer that has a 64-bit processor
One of the following:
- 64-bit edition of Windows Server 2008 Standard with Service Pack 2 (SP2)
- 64-bit edition of Windows Server 2008 Enterprise with SP2
- 64-bit edition of Windows Server 2008 R2 Standard with SP1
- 64-bit edition of Windows Server 2008 R2 Enterprise with SP1
- Windows Server 2008 Datacenter RTM or later
- Windows Server 2008 R2 Datacenter RTM or later
Operating system for installing the Exchange management tools on a computer that has a 64-bit processor
One of the following:
- Windows Vista with SP2 for management tools only installation
- 64-bit edition of Windows Server 2008 Standard with SP2
- 64-bit edition of Windows Server 2008 Enterprise with SP2
- 64-bit edition of Windows Server 2008 R2 Standard
- 64-bit edition of Windows Server 2008 R2 Enterprise
- 64-bit edition of Windows 7