Your password isn't stored in a DLL, at least not as far as I'm aware. MSN Messenger uses Microsoft Passport to authenticate you with their servers. Your username (email) and password are stored on the Passport servers.
If you choose the "remember me" option when you login to MSN Messenger, your password is stored locally. I haven't looked to closely but I believe it is encrypted and stored in a cookie.
You can learn more about Passport validation at
www.passport.com. Their developer information tells you all the details of how login works. Basically, even web sites that use Passport for login (such as MSN.com or eBay), cannot "see" your password; that's entirely handled by Passport.
Jeff Hester
BigBlueBall.com