NT Virtual DOS Machine (NTVDM.EXE) in Windows 2000, NT and XP does not verify user execution permissions for 16-bit executable files, which allows local users to bypass the loader and execute arbitrary programs.
http://www.securityfocus.com/bid/5740
http://www.iss.net/security_center/static/10132.php
http://www.abtrusion.com/msexe16.asp
http://support.microsoft.com/default.aspx?scid=kb%3B%5BLN%5D%3B319458
http://archives.neohapsis.com/archives/bugtraq/2002-09/0211.html