Note that the truly hard part of software development is the specification and finding out from the users/owners exactly what it is they want (or at least think they want). Can you post as much detail as possible about what you are looking for and then I can comment in more detail. But from what you have said so far....

Whatever solution you choose it is probably not going to be trivial, I suspect it will be a few weeks of design/programming/testing rather than a few hours. An important factor is what PHP expertiese you have available. Note that moodle uses the php adoDB database layer which is modelled along the lines of some of the similar Microsoft library so that may make some of the coding easier.

I am assuming you are only intending to register the students automatically. I am assuming that the tutors will set up the moodle courses by hand. I make this assumption on the basis that courses are only set up infrequently and thus creating by hand should not be a hardship.

For the purpose of registering the students you could create a php script that interrogates the Admission systems through an ODBC driver to identify new students, extracts the details, e.g. firstname, lastname, and userid (assuming such a field exists) and pokes it into the moodle tables. Such a script might get kicked off by a cron job (automatically executed at set times).

That description glosses over a huge amount of details. For example what about generating a password, and what happens if there is no unique userid in the existing system to extract?

Disclaimer
Also I do not claim to be particularly expert at the underlying systems of Moodle and have only a passing familiarity with the user/authentication ssytem. There are consultants who specialise in moodle modifications and you may find out more from moodle.com (this is moodle.org)

It is hard to tell whether you should go SOAP, XML, or SeaMonkeys. That's the kind of decision you'll have to make yourself.

A few starting points:

- Check out the Authentication plugins, specifically the External Database and LDAP. These are mature and well used, though Moodle 1.5 is likely to include several improvements.

- Check out the Enrollment plugins, specifically the External Database. These are rather new. I suggest you wait a few days for 1.4.2 (which will include some important bugfixes). Moodle 1.5 will also contain an LDAP enrollment plugin which we have developed, and is in testing right now.