Was vetting through the vSphere 5.5 Upgrade Guide and found this excerpt :
vCenter Server 5.5 removes support for Windows Server 2003 as a host operating system. See the VMware Compatibility Guide at http://www.vmware.com/resources/compatibility/search.php and “Migrate vCenter Server and Components from a Windows Server 2003 Host,” on page 110.
vCenter Server 5.5 removes support for Windows Server 2008 SP1 as a host operating system. Upgrade Windows Server 2008 SP1 hosts to SP2 before upgrading vCenter Server to version 5.5. See the VMware Compatibility Guide at http://www.vmware.com/resources/compatibility/search.php and the Microsoft
Software Lifecycle Policy at http://support.microsoft.com/lifecycle/#ServicePackSupport.