For whatever reason, you may find that you need to bypass the Windows Server Udpate Services site in your organization to update your Vista machine. If you don't have ready access to Active Directory or your GPOs, there is a regkey you can flip that will allow your system to contact MS direct for your updates.

Here is the step by step:

1) Open your registry editor
2) Browse to HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\AU
3) Change the key UseWUServer from 1 to 0
4) Restart the Windows Update service
5) Voila! Update away...