Microsoft issued a critical patch for Internet Explorer today, described as protection against a “remote code execution” vulnerability. If your computer is set up for automatic updates, you should get the patch, well, automatically. You should also be able to run Windows Update (under Tools in IE). Or you can get the patch from http://update.microsoft.com/microsoftupdate/v6/default.aspx?ln=en-us
Microsoft usually issues security patches on the 2nd Tuesday of the month (Patch Tuesday), so this must be serious.