Last night an exploit was released affecting all Windows7 and Vista(Fully patched) servers. We have also confirmed in our lab that this exploit also works against windows server 2008.
The exploit allows for remote denial of service attacks against any of these server.
The bug lies in the SMB2.0. ( http://securityreason.com/exploitalert/7138 )
Microsoft has recommended that that the SMB port(445) be blocked until Microsoft releases a patch.
More can be read here: http://www.microsoft.com/technet/security/advisory/975497.mspx