Just a small (but significant :-P) correction to the above post:

sp_configure 'max degree of parallelism', 1

This issue appears in certain conditions (I've only seen it happening with certain statements using UNION ALLs) in which more than one processor (or processor core) is utilised for a query ("intra-query parallelism"). I understand it to be caused by the processes running over both processors deadlocking each other.

If you know the statements that are causing it, add the MaxDOP hint. That query will then be forced to operate on only one processor.

If you cannot locate it, then you can use the sp_configure 'max degree of parallelism' option but be warned that it means your whole SQL Server instance will be confined to only 1 processor (I think that is the scope of it - check BOL to be sure).