There is an on-going issue with windows update that affects some Vista users without really any known cause or solution. http://www.vistax64.com/windows-updates/303992-windows-update-just-seems-hang-while-checking.html
There have been reports that manually downloading and installing this is a solution: https://www.microsoft.com/en-us/download/details.aspx?id=51871.
I wonder how far behind you are on Windows Updates? There will be more updates on Tuesday, so good luck catching up. Since you are trying to update Windows Defender, I assume that Microsoft Security Essentials is not installed, is that correct?