Today's the second Tuesday of the month, which means it's Microsoft (and Adobe) Patch Day. One of Microsoft updates fixes a zero day vulnerability, so we recommend you install at least that one as quickly as possible.
According to their summary post for November 2013, Microsoft released eight security bulletins today, fixing 18 security flaws in products like Internet Explorer (IE), Windows, Office products, and Hyper-V. They rate three of the bulletins asCritical.
The most critical update is the ActiveX one, since it fixes a zero day flaw. A few days ago, researchers at FireEyereported that advanced attackers were exploiting a previously unknown IE flaw in targeted attacks. Microsoft quicklyconfirmed the flaw was due to a particular ActiveX control, and promised to fix it today. Since attackers are exploiting this particular ActiveX control in the wild, you should apply the ActiveX "killbit" patch first. The IE and GDI updates also fix some pretty serious issues, so I would apply those patches quickly as well.
For those wondering, Microsoft hasn't yet released a patch for the previously reported zero day TIFF vulnerability. If you haven't installed the FixIt I recommended last week, be sure to do that too.
In a nutshell, check out Microsoft's summary and try to install all the updates at your earliest convenience. Microsoft's Auto Update can make the process easier, but I still recommend you test server-related updates before applying them.