What is SQL Server 2000 spid -2?
From the documentation on sp_who:


  In SQL Server 2000, all orphaned DTC transactions are assigned the SPID value of '-2'. Orphaned DTC transactions are distributed transactions that are not associated with any SPID. Thus, when an orphaned transaction is blocking another process, this orphaned distributed transaction can be identified by its distinctive '-2' SPID value. For more information, see KILL.


You can find the actual owner of the work using:

USE master;
GO
SELECT req_transactionUOW -- UOW GUID
  FROM syslockinfo 
  WHERE req_spid = -2;


You can then use:

KILL 'GUID_from_above';