Verify that all components meet basic compatibility requirements before upgrading vCenter Server.
Upgrading the operating system of a vCenter Single Sign-On 5.1 machine from Windows 2003 to Windows 2008 to meet the operating system requirements may result in symptoms similar to knowledge base article 2036170.
Prerequisites
Verify that your system meets the hardware and software requirements. See vCenter Server for Windows Requirements and vCenter Server Appliance Requirements
If you have solutions or plug-ins, check the VMware Product Interoperability Matrix. See http://www.vmware.com/resources/compatibility/sim/interop_matrix.php