not be able to access mapped drives

I have a few XP systems that loose their ability to access shares/mapped drives. Our network layout is as this. We have three servers 2x 2003 and 1x 2000 at our Head Office and at the remote location where this issue occurs we have a NT4.0 Server. Some times they log in and the mappings work and somtimes they don't. One of the errors that come up when you click on a mapped drive is. It will say L:\ (or what ever drive you are trying) is not accessble. Followed by this message "The system detected a possible attempt to compromise security. Please ensure that you can contact the server that authenticated you". I can not find out why this happens and only happens somtimes. More then not though.