One of the more frustrating parts of operating a modern computer is when the update/patch process fails. In the case of Windows, you very quickly fall into the bowels of system administration, with nothing but cryptic 16 digit hexadecimal error codes to guide you.
But there is one trick that works more often than not. It’s also one that I tend to forget, and only rediscover after a fair amount of hair-pulling:
You have to enter this at an administrator-level command prompt (in Win7 click Start, type cmd in the “find app” box, right-click the cmd.exe result and select “Run as Administrator”). And it takes a L-O-N-G time to complete. But it can fix many system files whose corruption is a common cause of the “update failed to install” error.
You can read more about this at http://support.microsoft.com/kb/929833.