Changing Passwords using Directory Services

Greetings!

A little background: I've been charged to write a vb.net app that will accept a user name, an old password, and a new password and when the user clicks a button, the password for that user is changed on our AS/400 as well as in Active Directory. Changing it on the 400 is a piece of cake. However, I haven't had much luck on the latter. I've tried API calls but I can't seem to get them to work. If anybody could offer some direction I would appreciate it! I'm somewhat familiar with the DirectoryServices namespace, such as getting properties out, but I haven't found anything regarding changing user passwords. We run XP Pro and Win2k clients and Win2k Server.

Again any help would be most appreciated

Thanks in advance!!!!!!!!