No login prompt in file sharing among XP and Win2000

I have setup a XP Pro machine with file sharing, simple sharing
disabled, ACL and sharing permission should be setup correctly.
However when I using my win2000 machine to connect to the XP, there is
no Login Prompt for me to key in the username and password. The only
error message is "Access is denied".

I have used the same sharing mechanism in 2 w2k machines for a long
period. If the client username or password doesn't match with the
host, a login prompt should appear, however this not case if I chnage
to WinXP.

The only solution I used is to map the host with a drive letter and
specify "Connect using a different user name". But my company have
more than 1000 staff, I can't tell every of them with such complicated
task.

I also referenced the article "Windows XP Professional File Sharing"
by Ron Lowe and Steve Winograd"
(http://www.practicallynetworked.com/...ring/index.htm),
however, I can't make the login prompt apppear.

Please help.
Regards,
Alex Tang