On Monday, Microsoft issued an Out of Band Security Update for Windows 7, Vista, and Windows XP (SP3) to stop the spread of a vicious new worm taking advantage of a vulnerability in Windows LNK (shortcut) files. The vulnerability is already being used by malware. The update (KB2286198) will automatically install if you have Windows Update enabled. If you don't currently use Automatic Updates, you need to install the patch without delay.
To update Windows 7 or Vista manually, go to Start Menu, All Programs, click Windows Update, then click the link to Check for updates: