Cant access network drive

Hi,
Hel me please!

Server:Win 2003 (no domain)
Workstations: about 100 Win XP Prof SP3
We have a share resource in the server: T drive (the same login for all)
From last weekend four users (acounts) cant access the T drive, the other ones yes. If we create new user account in the PC, we can access.

Note: When I map the drive, I can see it but I cant access.

Regards,
Alexis Lara