I have used Microsoft's Security Essentials on Windows 7 and Windows Defender on Windows 8 - they are differently-branded versions of essentially the same antivirus/antimalware software. The programs work quite well in general, but I have seen occasions where the update process has problems. This article on 404 Tech Support describes the issue and a solution for Windows Defender, and in fact Microsoft has a "Fit It" for it.
404TechSupportI hit the Update button for Windows Defender and it seemed to take a little longer than normal but the progress bar continued to advance. Eventually, it would return to the Home tab and show the message Connection failed next to ‘Virus and spyware definitions’.