Microsoft released the announced out of band security update MS08-067. Please read the bulletin carefully and then apply the update as soon as possible.
http://www.microsoft.com/technet/security/Bulletin/MS08-067.mspx

In addition there is also an excellent post with a lot more details on the vulnerability Microsoft patched. You should definitely read it: http://blogs.technet.com/swi/archive/2008/10/23/More-detail-about-MS08-067.aspx

Urs