You can manually opt-in to Microsoft Update (allowing installation of updates for other Microsoft products through Windows Update) through a simple script:

VBScript

ServiceManager = CreateObject("Microsoft.Update.ServiceManager")
ServiceManager.ClientApplicationID = "My App"
Set NewUpdateService = ServiceManager.AddService2("7971f918-a847-4430-9279-4a52d1efe18d",7,"")

Powershell

$ServiceManager = New-Object -ComObject "Microsoft.Update.ServiceManager"
$ServiceManager.ClientApplicationID = "My App"
$ServiceManager.AddService2("7971f918-a847-4430-9279-4a52d1efe18d",7,"")

Note that you probably need at least version 7.0.6000 of Windows Update Agent. This can usually be obtained by installing the regular Windows updates first, all the way through to the end.