Mapped Network Drive Errors

I am using SBS 2003 with seven XP clients. Each client has a mapped drive. They have worked fine for several months without any problem. Since switching our ISP two weeks ago, file sharing has been acting up.

For example. Client A will be able to access Client B and C's mapped drives but will receive a "local device name already in use" error when trying to connect to Client D. However, all other clients will be able to access Client D's mapped drives without any trouble. All clients have the same share settings.

Also, networked printers suddenly go offline for no reason but maintain a "ready" status.

Any ideas? Any help would be greatly appreciated. Thanks.