Alguns artigos úteis da Microsoft:
How to Troubleshoot Windows Update
Microsoft Windows Update Troubleshooter (use Internet Explorer).
Se você se considera bem informado sobre o uso do Windows, sua melhor aposta é provavelmente usar o utilitário gratuito Dial-a- corrigir :
Dial-a-fix (hereafter known as "DAF") is a collection of known fixes gleaned from Microsoft Knowledgebase articles, Microsoft MVPs, and other important support forums, that will assist you in repairing problems with your system. Although this tool is ordinarily meant for power users, technicians, and administrators, it is quite safe to use even without technical guidance (although guidance is recommended). Simply choose the solutions you wish to apply via checkmarks, and click GO. There are other buttons and tools present on the main dialog as well, such as the policy scanner. All tools and checkmarks identify their purpose when you mouse over them.
Veja este artigo para mais informações.