FIX: An access violation may occur, and you may receive an error message, when you query the sys.dm_exe_sessions dynamic management view in SQL Server 2005
Source: Knowledge Base
Product: Microsoft SQL Server 2005 Enterprise Edition & SQL Server 2005 Standard Edition
Notification Contents: New and Major Modifications
FIX: An access violation may occur, and you may receive an error message, when you query the sys.dm_exe_sessions dynamic management view in SQL Server 2005
http://support.microsoft.com/kb/940281/en-US