Two problems with the code right off:

First, the username should be specified as "CN=xxxx", as the Add method with
the LDAP provider takes a relative distinguished name:

Dim NewUser As DirectoryEntry = AD.Children.Add("CN=TestUser1", "user")

Second, yu should save the user first before trying to set the password as
the object needs to be persisted to the directory before the password can be
set:

NewUser.CommitChanges()
NewUser.Invoke("SetPassword", "NewPassword")

Depending on whether the domain is AD 2000 or 2003, you may need to set the
sAMAccountName attribute before you do the initial save.  2003 will create a
default value, but 2000 will not.  Note that using the default value is
probably a bad idea as that will essentially give the user a semi-random
logon name.  I recommend always setting it explicitly.

NewUser.Properties("sAMAccountName").Value = "TestUser1"

If you want to make sure you use SSL to bind to the directory, make sure
AuthenticationTypes.SecureSocketsLayer is set on the parent object before it
is bound to the directory.  Note that your AD must be configured with an SSL
certificate to use SSL/LDAP.

HTH,

Joe K.