So when PostgreSQL runs "copy 000...5E D:\...", it fails, and when you do the same thing as the PostgreSQL user, it works. Interesting. Try increasing the log level in postgresql.conf to see if it logs the error message from copy, or try xcopy instead of copy.
Note thatn when PostgreSQL runs, it will shed any rights given through "Administrators" or "Power Users" group. So this is not an identical test.
We cannot assume that the one-click installer was used, but if it was, the service account it creates will be a member of the Users group only.
tuanhoanganh, what did you download to install Postgres?