Deadlocking occurs when two user processes have locks on separate objects and each process is trying to acquire a lock on the object that the other process has. When this happens, SQL Server ends the deadlock by automatically choosing one and aborting the process, allowing the other process to continue. The aborted transaction is rolled back and an error message is sent to the user of the aborted process. Generally, the transaction that requires the least amount of overhead to rollback is the transaction that is aborted.

Link per problematiche deadlocks

http://www.sql-server-performance.com/deadlocks.asp; http://msdn2.microsoft.com/en-us/library/ms187101.aspx

Consider turning on SQL Server deadlock tracing in order to track deadlocks, when you are having deadlock-related issues. The overhead for doing this is minimal.

DBCC TRACEON (3605,1204,-1)

One way to help prevent deadlocks is to use the UPDLOCK optimizer hint. What this hint does is to force SQL Server to use an update lock instead of a shared lock.