With Kixtart you can write to the registry with "WRITEVALUE".
So you can change the windows update url to the url you want.
On my Windows2000 Pro machine the url can be found in HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\WindowsUpdate.
I don't no or it's different by other windows versions.
But if you search for WindowsUpdate in the registry I'm sure you can found the key.