I would recommend you to check date/time. Clean your antivirus with its removal tool (Also clean the all others you have previously installed, if any). Then again install your antivirus. After that I think windows update will work. Make sure to clean all
antiviruses. You can also use AppRemover (http://www.appremover.com) for this purpose.
List of removal tool of antivirus can be found here:
http://experts.windows.com/w/experts_wiki/156.aspx