There's definitely a method in the ADSI stack somewhere to change the
password on a given user - provided it's on an LDAP server (Active
Directory server - not a local server user).

Not sure about the NetValidateName - what are you trying to
accomplish? You can definitely search the AD object hierarchy for any
user and see if it exists - yes. Is that what you're looking for?

More info and code samples can be found here:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/sds/sds/portal.asp

Marc