Read more information from this ZDNet article:
Microsoft releases emergency patch for all versions of Windows
If you haven't updated yet, your PC could be taken over simply by visiting an infected web page. So here's what to do:
To make sure you are up-to-date, close all your open windows, then click the start button and see if there is an update indicated on the Shut Down button. If you hover your mouse over it, it will say "Update and shut down" or "Update and restart". If so click it and get it done and you won't have to worry.
If no update seems to be present, go into the Control Panel (hold the Windows key and tap I to find the control panel) and open the item called "Windows Update" found under the System and Security section. If there are updates present, click the "Install Now" button. If not, click the link in the left column that says "Check for updates". It will take a few minutes then if updates are will provide you with the "Install Now" button.
There have been a few of these types of flaws in the past, but they're really not that common.