Unix & Windows passwords have to be the same ??

Ok. First things first, I'm not sure if this is the best place to put this question, but here goes.
I have a windows 2000 AD domain (which I administer). We also have a Unix Database server (SCO openserver release 5.5) (another unix administrator). I am not to unix friendly but he is. We have several employees that have mapped drives to a directory on the unix system. Everything works fine but here is my problem. The windows username and password MUST match UNIX's username and password in order for them to be able to read that directory. Lets say If they happen to change there windows password, then they can no longer access that mapped directory on unix. I have to manually update the unix username/password for that user. Is this something that can be changed or fixed ? Another way around this ? I appreciate all the help, Thank you in advance.