Slow Sql Update MS SQL 2000

I have the following statement that takes quite a long time. Longest
of any of my SQL statment updates.

UPDATE F_REGISTRATION_STD_SESSION
SET PREVIOUS_YEAR_SESSION_ID = (
SELECT s.previous_year_session_id
FROM F_REGISTRATION_STD_SESSION R
,DS_V4_TARGET.dbo.D_H_Session_By_SessQtr S
WHERE
r.STUDENT_ID = F_REGISTRATION_STD_SESSION.STUDENT_ID
and s.previous_year_SESSION_ID = r.SESSION_ID
and s.session_id = F_REGISTRATION_STD_SESSION.SESSION_ID
)
STUDENT_ID varchar(20) and SESSION_ID char(10) and are indexed
What I want to accomplish:
I want to know if there was a student registration from the prior year
of a registration.
Example, if there is a registration for Fall 2004, was there also a
registration for the same student Fall 2003?
Maybe there is a better way to approach this?
TIA
Rob