How Can I Get My Sproc To Return Records With 0 As ClientID

Can anyone help me modify this sproc's Where clause or Joins to let (T) task records with a 0 to be returned?

If I enter a ClientID I want to return only those task records with the ClientID I entered (this works).

If I enter no ClientID I want to return all task records, even those with a 0 in the ClientID field.

ALTER PROCEDURE dbo.CMAdmin

@SID int


AS
SELECT
 A.CompanyName,
 C.FirstName,
 C.LastName,
 C.ClientID,Convert(varchar(10),
 T.ActionDate, 10) AS [Action Date],
 T.Priority,
 T.Status,
 T.Subject,
 T.Note,
 T.CompletionDate, 10) AS Completed,
 T.DateEntered AS Entered,
 T.EnteredBy AS [Entered By],

CASE
WHEN A.[CompanyName] IS NULL OR A.[CompanyName] = '' THEN
 C.[FirstName] +' '+ C.[LastName]
 ELSE A.[CompanyName]
 END AS DRName

FROM
tblClients C LEFT OUTER JOIN tblClientAddresses A
ON C.ClientID = A.ClientID LEFT OUTER JOIN dbo.tblTasks T
ON C.ClientID = T.ClientID

WHERE
C.ClientID =  Isnull(@SID,C.ClientID)