The latest round of virus attacks have been exceptionally dangerous. Appearing as signed official installers, these trojans can wreck a windows machine, requiring a complete reinstall of Windows. By mucking with the digital certificates used by Microsoft and others to verify updates/installers, the virus can render a Windows machine unable to update itself in anyway, even after the virus has been completely cleaned off. We have seen many machines with this infection since Thanksgiving. So please be careful and stay aware of what you are clicking or installing!

ZeroAccess Rootkit Launched by Signed Installers | Blog Central
A few days ago, we first saw a new attack that turned out to be variants of the infamous ZeroAccess rootkit, launched by digitally signed installers and uninstallers. In the cases observed so far, the signed application is a valid program–such as the installer for recent Flash Player versions