could not open file xxxx for writing: Permission denied

Hello,

Using PostgreSQL 8.1.4, pgadmin III 1.6.0 in WinXP I tried to export a table
using COPY (first time in postgresql, did many times in Sybase):

COPY ml50jtsk_datum_v TO 'c:/postgresql/ml50jtsk.out';

I got the following errormessage:

ERROR: could not open file "c:/postgresql/ml50jtsk.out" for writing:
Permission denied
SQL state: 42501

What's wrong? Does it mean that the database user has no writing permission
out of the database? How I did a pg_dump then few weeks ago?

Many thanks for help and/or explanation, best regards and Happy New Year

Tomas