Here is a short script that will install the prerequisites for System Center Virtual Machine Manager 2008:
ServerManagerCMD.exe -i PowerShell
ServerManagerCMD.exe -i Web-Server
ServerManagerCMD.exe -i Web-Asp-Net
ServerManagerCMD.exe -i Web-Metabase
ServerManagerCMD.exe -i Web-WMI
Since ServerManagerCMD.exe is deprecated in Windows Server 2008 R2 and has been replaced with PowerShell cmdlets, here is the PowerShell command as well:
Remember to do ImportSystemModules or Import-Module Servermanager first.
No need to do PowerShell since it is already installed on Windows Server 2008 R2.
Also, remember that SCVMM needs the Domain Functional level to be at least 2, that is Windows Server 2003. The Configuration Analyzer will warn you of this: