Question : Can two users log on to one computer at the same time?

Server: window SB 2003
Client computer: window XP Pro.

I want to use remote desktop connection to connect to a client computer, which is being used by another user. Is it possible we can log on at the same time?

Answer : Can two users log on to one computer at the same time?

If you were correct in your original comment "I want to use remote desktop connection to connect to a client computer" the answer is no.  Windows XP (as you stated was your client computer) does not support two logins being *USED* by two humans at the *same time*.
(except via "Remote Assistance" which means you are shareing "ONE" session, without 3rd party software)

http://support.microsoft.com/default.aspx?scid=kb;en-us;279656&sd=tech
"However, in Windows XP Professional you cannot create a shadow session, where a local user and a remote user can control the same session."  This is also the case for seperate session in Windows XP.