Although in my WINNT.SIF i have AutomaticUpdates="Yes", after the installation finishes, i get the following window.

i had a similar problem, automatic update were enabled like you have but when i would check the status, they would not be enabled or disabled, neither checked. well in order to set a time for it to check, i searched my registry and luckily found what i was looking for.

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\Auto Update]
"AUOptions"=dword:00000004
"ScheduledInstallDay"=dword:00000000
"ScheduledInstallTime"=dword:00000003

you can throw that into your installation somehwere, it sets windows update to check every night at 3am. if you want specific settings, i suggest change them manually then check the registry settings and use whatever results you get.

Ok, I'll give it a try, thanks