Here's a sample .reg file I carry on my USB flash drive:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate]
"TargetGroup"="XXXXXX"
"TargetGroupEnabled"=dword:00000001
"WUServer"="http://wsus.mydomain.com:8530"
"WUStatusServer"="http://wsus.mydomain.com:8530"
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\AU]
"AUOptions"=dword:00000004
"NoAutoUpdate"=dword:00000000
"ScheduledInstallDay"=dword:00000000
"ScheduledInstallTime"=dword:00000002
"UseWUServer"=dword:00000001
I believe the minimum options are:
WUServer
WUStatusServer
UseWUServer
AUOptions
http://technet.microsoft.com/en-us/library/cc708449.aspx