[Laurent Gaffié] has discovered an exploit that affects Windows Vista, Windows 7, and possibly Windows Server 2008 (unconfirmed). This method attacks via the NEGOTIATE PROTOCOL REQUEST which is the first SMB query sent. The vulnerability is present only on Windows versions that include Server Message Block 2.0 and have the protocol enabled. A successful attack requires no local access to the machine and results in a Blue Screen of Death.
[Laurent] has a proof of concept available with his writeup in the form of a python script (please, white hat use only). There is no patch for this vulnerability but disabling the SMB protocol will protect your system until one is available.
Update: According to the Microsoft advisory this vulnerability could lead to code execution, making it a bit worse than we thought. On the bright side, they claim that the final version of Windows 7 is not open to this attack, only Windows Vista and Windows Server 2008.
[via Full Disclosure]