Microsoft usually releases patches for Windows on the second Tuesday of the month. But last Thursday, the company sent out an update that was too important to withhold for two more weeks. If you have your PC set to download and/or install Windows updates automatically, you've already received this patch. (You'll find more on managing Windows updates at this previous Worker's Edge.)
In Vista, you can make sure you've got this update by pressing the Windows key, typing Windows Update, and pressing Enter. Click "View update history" in the left pane and look for an entry labeled Security Update for Windows Vista (KB958644) and dated 10/23/2008.
The patch is even more important for Windows XP PCs, which lack some of Vista's built-in defenses, such as User Access Control (yes, it's good for something). You can open Windows Update by clicking the applet's shortcut on XP's Start menu, or assign the shortcut a key combination to open it from your keyboard .
To do so, right-click the Windows Update shortcut on the Start menu, choose Properties, click in the "Shortcut key" box under the Shortcut tab, and press your desired key combination--I chose Ctrl-Alt-U.
In XP's Windows Update applet, click "Review your update history" on the left and look for an entry with the Knowledgebase (KB) number 958644 (see the screen above).
If you have trouble downloading or installing this or any other Windows update, see Scott Dunn's information on troubleshooting Windows Update. (Scroll down to "Tips for installing recalcitrant updates" about halfway through the article.)