Stored Proc From 2000 To 2005

Hi, I have the following stored procedure working in SQL 2000:


(
@p_ServiceID INT = NULL
)

AS

SELECT
S.ServiceID,
.............
.............

FROM
tblServices AS S
WHERE
(@p_ServiceID = NULL OR @p_ServiceID = S.ServiceID)


This returns all services if a null or nothing is passed, and returns a specific service if a proper service ID is passed.

However, in 2005 this doesn't work. If I pass a service ID, it returns it, but if I pass nothing or a null, it returns no rows.

Any ideas?

Thanks,
James