If you are using windows vista on a domain you may find you are unable to get windows updates.
When added to a domain Vista adds a registry key (a policy) to look for a WSUS server. I don't know the ins and outs of it but I presume that if that server isn't dishing out Vista updates it fails.
So anyway the key to delete is...
HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\WindowsUpdate
there is a subkey called AU; delete that as well!
Restart windows and set the update going and all should be grand! :)