Microsoft announced that the Windows 7 / Windows Server 2008 R2 Service Pack 1 updates available on WSUS have been revised to ensure that these updates are installed exclusively. This change has been made in response to the issue described in KB975484.
"Previously, Service Pack 1 could be installed at the same time as other updates which was reported to cause problems during the installation of Service Pack 1 for some customers. While this change does not fully resolve the issue for customers who have encountered the problem, it will help prevent it from occurring," informs Microsoft.
Note that administrators should ensure that their managed machines are rebooted after installing Service Pack 1 and before scanning for or installing other updates.
Please see KB975484 for the latest information.
[source: WSUS Product Team Blog]