After unsuccessfully going through the fixes Microsoft suggested to solve the problem, I uninstalled BitDefender Antivirus v10 and Windows Update worked. I contacted BitDefender Support:
> In Vista, I had to uninstall BitDefender Antivirus v10 to get Windows
> Update to work. Is there a fix?
and received this enigmatic response:
Dear XXXX,
Thank you for your interest in our BitDefender security solution.
Please tell us, are you using Windows Vista ?
Best regards,
XXXX
BitDefender Technical Support Engineer
That was three days ago. BitDefender Antivirus v 10 is not "Vista Ready" until it is compatible with Windows Update.