pg_dump.exe is failing with a fatal error

Hi,

I am trying to take a backup of the database in windows 2000 server on
cygwin environment.
The command used is pg_dump.exe -O -d dbname > sampleDB
error message : pg_dump: [archiver (db)] connection to database "dbname"
failed : FATAL 1: user "Administrator" does not exist

I am using User "Administrator" only for my activity.

Any Idea/suggestion ?

Your response will be highly appreciated.

Thanks,
Abhijeet