How do you compare (NULL) and (non-NULL)?

In the following query

SELECT Parent FROM Channels ORDER BY Parent ASC;

If I have a couple of (NULL)s in the field [Parent], they will be listed at
the bottom of the query result.

Is it because PostgreSQL considers (NULL) as the biggest value? If I run the
same query under MSSQL Server 2000, I get the exact opposite result
regarding the order of (NULL)s and (non-NULL) values. They are listed at the
very beginning of the query result.

Thanks

Wei