How to determine which statement is blocking an update in SQL Server
Before you run your update:

SELECT @@SPID;


Now, run the update, if it is not completing quick enough, then in another window run:

SELECT status, command, wait_type, last_wait_type, blocking_session_id
  FROM sys.dm_exec_requests
  WHERE session_id = &lt;@@SPID from above&gt;;


If you get a value in blocking_session_id, then you can run the same query for that SPID. You can also say:

DBCC INPUTBUFFER(&lt;blocking spid&gt;);


(Well, as long as you're not on SQL Azure.)

If this doesn't yield a clue, then it's possible the blocking statement was actually at a different point in a larger transaction. You'll have to track that down by figuring out where that spid is executing from and whether it's a batch of ad hoc T-SQL, a stored procedure, etc. There's not really any magic "look here!" we can provide you.

If it's not getting blocked, you may find relevant info in the other columns that can help determine why it's slow (e.g. perhaps it is waiting on a ginormous log autogrow event).