Yesterday Microsoft released a critical Windows security patch outside of their normal “patch Tuesday” schedule. This one is important as it patches a vulnerability that has been exploited in the wild. If you have automatic updates turned on then you should have it already, if not, download it and install it now. It affects all versions of Windows.
You can read more about it here: http://www.microsoft.com/technet/security/bulletin/ms08-067.mspx