Time Delay In SQL Server 2000 For Date Variable

Hello I have and tested in 2 way using variable and constant.
Query 1: This query I am using vaiable "@last" for the date value.
this takes to run for 2 minute .

Query 2: Here I am using value '11/05/2002' instead of varialbe and it is running within a second.

Why these much difference to run this same query?.
Can anybody give correct reason or solution for this.
Thanks,
Ravi

query 1:
DECLARE @last smalldatetime
SET @last ='11/05/2002'

select distinct(o.account_num),a.enterprise_num,a.site_nu m
from oesd.dbo.order_header o WITH(NOLOCK)
INNER JOIN oesd.dbo.account a WITH(NOLOCK)
ON a.account_num = o.account_num
and o.added_date > @last
UNION
select distinct(o.account_num),a.enterprise_num,a.site_nu m
from oesd.dbo.order_header o WITH(NOLOCK)
INNER JOIN oesd.dbo.account a WITH(NOLOCK)
ON a.account_num = o.account_num
INNER JOIN Leap.dbo.ENTERPRISE_STATE es WITH(NOLOCK)
ON (a.enterprise_num = es.ENTERPRISE_NUM
AND o.last_updated_date > @last)


Query 2:

select distinct(o.account_num),a.enterprise_num,a.site_nu m
from oesd.dbo.order_header o WITH(NOLOCK)
INNER JOIN oesd.dbo.account a WITH(NOLOCK)
ON a.account_num = o.account_num
and o.added_date > '11/05/2002'
UNION
select distinct(o.account_num),a.enterprise_num,a.site_nu m
from oesd.dbo.order_header o WITH(NOLOCK)
INNER JOIN oesd.dbo.account a WITH(NOLOCK)
ON a.account_num = o.account_num
INNER JOIN Leap.dbo.ENTERPRISE_STATE es WITH(NOLOCK)
ON (a.enterprise_num = es.ENTERPRISE_NUM
AND o.last_updated_date > '11/05/2002' )