Mapped Drives Do Not Appear at Login

We have several users who, when they log in, do not have the drives mapped per their login script. Well, some times only one drive is missing; other times both drives are missing. A reboot usually clears the problem. These folks are on Windows XP w. the latest patches. The mapped drives (2) are mapped to different shares on a single Windows 2003 server (running the latest patches). I’ve searched MS Support KB and not found an answer and seen things all over the map when doing a Google search on this.

Many thanks for your help!