If you’re using Microsoft’s free “Security Essentials” anti-virus suite, you can update your virus definitions in just a few seconds by following the right steps. In the box that appears, type “Security Essentials” into the “Search” box.
It secured AV-TEST certification on October 2009, having demonstrated its ability to eliminate all widely encountered malware.
Running a virus scan on your computer with outdated definitions is essentially as useful as not running a virus scan at all.
Else you could also run Windows Update in Clean Boot State and see if that works for you.
Tip in tip: How to manually update Microsoft Security Essentials.
AVG, Mc Afee, Sophos and Trend Micro claimed that the integration of the product into Microsoft Windows would be a violation of competition law.