解决方法：
在查询分析器里面，选择windows验证，进去，然后执行下面语句，然后再去修改，就OK了。以后不用重装数据库了。

在查询分析器执行以下代码：

create procedure sp_password
@old sysname = NULL, -- the old (current) password
@new sysname, -- the new password
@loginame sysname = NULL -- user to change password on
as
-- SETUP RUNTIME OPTIONS / DECLARE VARIABLES --
set nocount on
declare @self int
select @self = CASE WHEN @loginame is null THEN 1 ELSE 2 END

-- RESOLVE LOGIN NAME
if @loginame is null
select @loginame = suser_sname()

-- CHECK PERMISSIONS (SecurityAdmin per Richard Waymire) --
IF (not is_srvrolemember('securityadmin') = 1)
AND not @self = 1
begin
dbcc auditevent (107, @self, 0, @loginame, NULL, NULL, NULL)
raiserror(15210,-1,-1)
return (1)
end
ELSE
begin
dbcc auditevent (107, @self, 1, @loginame, NULL, NULL, NULL)
end

-- DISALLOW USER TRANSACTION --
set implicit_transactions off
IF (@@trancount > 0)
begin
raiserror(15002,-1,-1,'sp_password')
return (1)
end

-- RESOLVE LOGIN NAME (disallows nt names)
if not exists (select * from master.dbo.syslogins where
loginname = @loginame and isntname = 0)
begin
raiserror(15007,-1,-1,@loginame)
return (1)
end

-- IF non-SYSADMIN ATTEMPTING CHANGE TO SYSADMIN, REQUIRE PASSWORD (218078) --
if (@self <> 1 AND is_srvrolemember('sysadmin') = 0 AND exists
(SELECT * FROM master.dbo.syslogins WHERE loginname = @loginame and isntname = 0
AND sysadmin = 1) )
SELECT @self = 1

-- CHECK OLD PASSWORD IF NEEDED --
if (@self = 1 or @old is not null)
if not exists (select * from master.dbo.sysxlogins
where srvid IS NULL and
name = @loginame and
( (@old is null and password is null) or
(pwdcompare(@old, password, (CASE WHEN xstatus&2048 = 2048 THEN 1 ELSE 0 END)) = 1) ) )
begin
raiserror(15211,-1,-1)
return (1)
end

-- CHANGE THE PASSWORD --
update master.dbo.sysxlogins
set password = convert(varbinary(256), pwdencrypt(@new)), xdate2 = getdate(), xstatus = xstatus & (~2048)
where name = @loginame and srvid IS NULL

-- UPDATE PROTECTION TIMESTAMP FOR MASTER DB, TO INDICATE SYSLOGINS CHANGE --
exec('use master grant all to null')

-- FINALIZATION: RETURN SUCCESS/FAILURE --
if @@error <> 0
return (1)
raiserror(15478,-1,-1)
return (0) -- sp_password