Passing new parameter to stored procedure

Hi Experts,

I have 1 reporting database in SQL 2000, developing front end in VB.net
user passes 3 parameters to back end

Start date
Enddate
Dept
Dept value is picked by combo box binded to table field .
i have enetered value " All" in combo fields along with other fields coming through data binding.

when user enters start date, End Date, and from combox selected value is " All"

my store procedured at the back end will pull the data  for all departments based on given date range.

my question starts here.

if user gives start date  end date , and any specific dept name.
i just need to pull the information based on the given range as well as that dept only.

right now i have all togather 8/9 Sps. in whuch i am passing SDate, EDate and 1 criteria ID.

i need something like if statement here which can distinguish the select statements that . if value is coming "All" then pull all information ased on given dates or if value is other than "All"( which means user picked some dept) then pull information based on that date range , criteria id and that specific dept.
Example Sp1 is here

sp_CSales @SD  smalldatetime,@ED smalldatetime,@c int
AS

DECLARE @S AS  int
DECLARE @E AS  int

 SELECT @S = '1'+ CONVERT(VARCHAR(20),@SD,12)
 SELECT @E = '1'+ CONVERT(VARCHAR(20),@ED,12)
   


INSERT  into CSALES (CID,Loc,Cost,Sells,Qty,STNAMS,DPDPNM,Amt$,F1DEPT,F1DATE)
 

  SELECT   @c,store.LOC, SUM(Sales.SumOfF1CV) AS Cost, SUM(Sales.SumOfF1SV) AS Sells, SUM(Sales.SumOfF1QTY) AS Qty,
                      store.STNAMS, Sales.DPDPNM, Sales.GP$ AS Amt$, Sales.F1DEPT, Sales.F1DATE
FROM         store INNER JOIN
                      Sales ON store.LOC = Sales.F1LOC
GROUP BY store.LOC, store.STNAMS, Sales.DPDPNM, Sales.GP$, dbo.Sales.F1DEPT, Sales.F1DATE
HAVING      (Sales.F1DATE >=@S AND Sales.F1DATE <= @E)
ORDER BY store.LOC





Regards
Bilal