If you are using any version of Internet Explorer, you need to apply a security patch immediately.

Microsoft has discovered a previously unknown security flaw in every version of Internet Explorer since 5.01 that will allow hackers to execute remote code on your computer.  In every day terms this means that just visiting a web site with malicious code installed could push a malicious program in to your system and take control of your system, steal personal information, use your system to send out spam emails and more.

Here is what Microsoft has to say about the situation from their security bulletin:

This security update also resolves three privately reported vulnerabilities in Internet Explorer. These vulnerabilities could allow remote code execution if a user views a specially crafted Web page using Internet Explorer. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.

This security update is rated Critical for Internet Explorer 5.01 and Internet Explorer 6 Service Pack 1, running on supported editions of Microsoft Windows 2000; Critical for Internet Explorer 6, Internet Explorer 7, and Internet Explorer 8 running on supported editions of Windows XP; Critical for Internet Explorer 7 and Internet Explorer 8 running on supported editions of Windows Vista; Moderate for Internet Explorer 6, Internet Explorer 7, and Internet Explorer 8 running on supported editions of Windows Server 2003; and Moderate for Internet Explorer 7 and Internet Explorer 8 running on supported editions of Windows Server 2008. For more information, see the subsection, Affected and Non-Affected Software, in this section.

The security update addresses these vulnerabilities by modifying the way that Internet Explorer handles objects in memory and table operations.

Let us stress that this is a CRITICAL UPDATE. Do not waste time, install this update immediately.  If you go and look at the chart they provided, you will see that there is no combination of Windows operating system and Internet Explorer since version 5.01 that is not impacted by this flaw.  (This seems like a good time to yet again mention, just change to a different browser that is more secure from step 1)