Well the first comment is I dread to imagine what nasties could be lurking on an OS that Microsoft ceased to support long ago.
Run ipconfig from a command prompt on both machines while connected by ethernet & compare the ethernet sections. Maybe you or a piece of malware has changed a setting. If all is well there I'd suspect that either the NIC has partially failed or you may have malware saturating the upload. Try downloading Malwarebytes or similar see if that throws up anything nasty.
