Method1:
You may run a windows update troubleshooter and check:
http://windows.microsoft.com/en-US/windows7/Open-the-Windows-Update-troubleshooter
Method 2:
You may reset windows update components. You may perform the steps in the link provided and check:
How do I reset Windows Update components?
http://support.microsoft.com/kb/971058
You may also refer the link:
Unable to install updates in Windows Vista, Windows 7, Windows Server 2008 and Windows Server 2008 R2
http://support.microsoft.com/kb/2509997
Also refer the link
How to troubleshoot Windows Vista and Windows Server 2008 service pack installation issues
http://support.microsoft.com/kb/947366
