We have a corporate WAN spread over most of the US, with a central HQ
employing several hundred people. I need a solution to create a
mailing list solution that can be accessed by everyone, that can also
be easily be updated and organized by the IT support staff when a user
is hired, moved, promoted, terminated, etc...

We are not currently on any sort of LDAP or MS Exchange system, nor is
the leadership planning on implementing one anytime soon. I would
prefer something that can be managed via a web interface, but am open
to other options. Ideally, this could be something that spits out a
list of email addresses that could be copied and pasted into an email
client when certain criteria are specified. For instance, it would be
great if we could  manage it in a manner similar to the way NT groups
and users are organized, like a user could be part of the "IT" group,
as well as the "Management" group.

Also, as a side-note: This solution needs to be able to run on a
2K/XP/WS2003 machine.

Hi,
I remember solving a very similar issue with MDaemon. That server has
a very powerful newsletter/mailinglist/digest engine that can be
totally controlled over SMTP.
Users send 'special command tags' to the server to do several actions.
All you need to do (your work) is to make available a simple static
webpage for the clients to learn about those 'special command tags'
and how to use them.
You will know that your users are able to totally manage their
accounts with your mailing server without any action required from any
thirs parties. I would like to tell also that the mentioned server has
a nice combinations of features and capabilities that will even help
the implementation with any LDAP server for example.

MDaemon can be found at www.mdaemon.com as far as i remember, wish you all luck.

www.sympa.org should help you. It works with Apache+ActiveState Perl
and that will all run nicely on a M$ machine.