What is recorded in the Workstation event logs, this will help isolate problem.
Just as guess: have you exceeded the connection licence count on the server, any new machines installed.??

>Thanks for your help Neil,
>
>The autodisconnect attribute in my registry was already set to 0xffffff.
>I already changed that a few days ago and it had no effect.
>
>When my drives disconnect, I cannot reconnect them back.. When Windows asks me for my username/password, it always says "invalid credentials" like if it was unable to reach the domain controller to validate my username/password.
>
>So I must logoff completely and login again to make this work. It is impossible to reconnect my drives.
>
>Do you think it can have something to do in the way our Primary Domain Controller is configured? Remember that everything works fine on Win2k workstations but not on WinXP workstations.
>
>Best regards,
> Alex