Bobbie Harder has recently published an excellent article about the WSUS Automatic Update Client.  Windows Server Update Services (WSUS) clients can be configured to provide update installation and reboot behavior best suited to your environment and your business needs. You can use Group Policy or Local Group Policy to modify Automatic Update configuration on your WSUS clients to determine what notification, download, install, and reboot behavior your WSUS managed clients will experience in updating from WSUS. Although there are policy settings for WSUS that control additional configuration, this article focuses on configuration options that define update notification, download, installation, and post-install reboot behavior.

For the remainder of the article, please see http://www.microsoft.com/technet/community/columns/sectip/st0506.mspx.