networking access problem – pls help!!!!

I have a server running Win2003 server. I’m trying to access its shared
folders from a WinXPPro SP2 computer. The username and password I use to log
into the WinXP computer is the same as a user I set up on the server. Why
does lately it always asks for a username and password when I try to access
the server from the workstation? Before it didn’t ask me for a username and password. I have other computers that don’t have the
pop-up box come up at all. Any idea why is this happening?