Make sure that your operating system supports vCenter Server. vCenter Server requires a 64-bit operating system, and the 64-bit system DSN is required for vCenter Server to connect to its database.
For a full list of supported operating systems, see http://kb.vmware.com/kb/2091273.
vCenter Server requires the Microsoft .NET 3.5 SP1 Framework. If it is not installed on your system, the vCenter Server installer installs it. The .NET 3.5 SP1 installation might require Internet connectivity to download more files.
If your vCenter Server host machine uses a non-English operating system, install both the Microsoft .NET Framework 3.5 SP1 and Microsoft .NET Framework 3.5 Language Pack through Windows Update. Windows Update automatically selects the correct localized version for your operating system. The .NET Framework installed through the vCenter Server installer includes only the English version.
vCenter Server 5.5 removes support for Windows Server 2003 as a host operating system. See the VMware Compatibility Guide at http://kb.vmware.com/kb/2091273.
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://kb.vmware.com/kb/2091273 and the Microsoft Software Lifecycle Policy at http://support.microsoft.com/lifecycle/#ServicePackSupport.
If you plan to use the Microsoft SQL Server 2008 R2 Express database that is bundled with vCenter Server, Microsoft Windows Installer version 4.5 (MSI 4.5) is required on your system. You can download MSI 4.5 from the Microsoft Web site. You can also install MSI 4.5 directly from the vCenter Server autorun.exe installer.
The VMware vCenter Server Appliance can be deployed only on hosts that are running ESX version 4.x or ESXi version 4.x or later.