Error In Query--datetime Related

Hi y'all I have a problem in my query.
This is my query
SELECT     dbo.BOS_GL_Workplace.WpszState, Emp2.szName, Emp1.szName AS szEmpName, Emp1.szEmployeeId,
                    dbo.BOS_PI_Division.szName AS Divisi, dbo.BOS_PI_Team.szDescription,dbo.BOS_SD_Route.szRouteId, dbo.BOS_SD_Route.szDescription, dbo.BOS_SD_Route.szScheduleId,
                    dbo.BOS_SD_RouteItem.szCustId, dbo.BOS_SD_Route.szOpUserId,dbo.BOS_AR_Customer.szCustId ,dbo.BOS_AR_Customer.szName, dbo.BOS_AR_Customer.CustszAddress_1 AS AlamatKirim, dbo.BOS_AR_Customer.szTaxId,
                    dbo.BOS_AR_Customer.CustszAddress_1 AS AlamatTagih, dbo.BOS_AR_Customer.CustszCity, dbo.BOS_AR_Customer.CustszZipCode,
                    dbo.BOS_AR_Customer.szStatus, dbo.BOS_AR_CustSales.szPaymentTermId, dbo.BOS_AR_CustSales.decCreditLimit,BOS_AR_Customer.dtmStart,BOS_AR_Customer.dtmLastUpdated
                    FROM         dbo.BOS_PI_Employee Emp1 LEFT OUTER JOIN
                    BOS_PI_Employee Emp2 on Emp1.szSupervisorID = Emp2.szEmployeeID Left outer join
                    dbo.BOS_GL_Workplace ON Emp1.szWorkplaceId = dbo.BOS_GL_Workplace.szWorkplaceId LEFT OUTER JOIN
                    dbo.BOS_PI_Division ON Emp1.szDivisionId = dbo.BOS_PI_Division.szDivisionId LEFT OUTER JOIN
                    dbo.BOS_PI_Team ON Emp1.szTeamId = dbo.BOS_PI_Team.szTeamId LEFT OUTER JOIN
                    BOS_SD_Route on Emp1.szEmployeeId = BOS_SD_route.szOPuserId LEFT OUTER JOIN
                    BOS_SD_RouteItem on BOS_SD_Route.szRouteId = BOS_SD_RouteItem.szRouteID LEFT OUTER JOIN
                    BOS_AR_Customer on BOS_SD_RouteItem.szCustID = BOS_AR_Customer.szCUstID LEFT OUTER JOIN
                    BOS_AR_CustSales on BOS_AR_Customer .szCustId = BOS_AR_CustSales.szCustID
When I run that query, it was working fine the result was correct. Then I tried to add the where clause, like this:
WHERE BOS_AR_Customer.dtmLastUpdated >= '8/1/2006' AND BOS_AR_Customer.dtmLastUpdated <='9/21/2007'
And it was also working fine, the result was also correct--pay attention dudes the date format is mm/dd/yyyy
Then I try to change the date format to dd/mm/yyyy in the where clause like this (because later I realized that, this is the query that gonna be used):
WHERE BOS_AR_Customer.dtmLastUpdated >= '1/8/2006' AND BOS_AR_Customer.dtmLastUpdated <='21/9/2007'
The result was an error message saying:
"The conversion of a char data type to a datetime data type resulted in an out-of-range datetime value."
Can you tell me whats wrong, please. I appreciate anykinds of help guys, thanks.
 
FYI : I use SQL SERVER 2000
 
Best Regards.