I was recently running some Windows Update patches on one of my client's Server 2003 boxes (they aren't using a patch management solution) when I ran into a strange error that stated the Windows Update site could not be accessed and the error message listed was 0x80070020.  This message did not appear while trying to install the updates, it appeared right after the selection for Express vs Custom.  I chose Custom and it's supposed to query the available patches for the server, but instead I was greeted with error 0x80070020.  In doing some reason on this error I found that it's related to a file locking issue.  Some people on various forums reported issues with BitDefender AntiVirus on Windows XP and Vista as the source of the issue.  While they're not running BitDefender on their server (they run eTrust), it was worth looking into.  I also found KB883825 which also lists Anti-Virus as a potential source of issues for Windows Update.  So I temporarily stopped the Realtime A/V scanner and voila!  Windows Update website began working again.