1. Create a file to store the connection password (Find the attached sample pgpass.conf)
        Refer: http://www.postgresql.org/docs/9.0/static/libpq-pgpass.html
2. Create a bat file with the following contents
        @echo off
        SET PGPASSFILE=D:\PgBackup\pgpass.conf

        for /f "tokens=1-4 delims=/ " %%i in ("%date%") do (
            set dow=%%i
            set month=%%j
            set day=%%k
            set year=%%l
        )
        set datestr=%month%_%day%_%year%

        set BACKUP_FILE=%datestr%.backup


        "D:\Program Files\PostgreSQL\9.0\bin\pg_dumpall.exe" --host=localhost --username=postgres --no-password --database=postgres --oids --file=D:\PgBackup\%BACKUP_FILE%
    Make necessary changes in the parameters and paths
        Refer: http://wiki.postgresql.org/wiki/Automated_Backup_on_Windows#Using_.pgpass_and_pgdumpall.2C_same_file
3. Schedule the bat file to execute at required time using windows task scheduler
        Refer: http://windows.microsoft.com/en-US/windows7/schedule-a-task

http://www.postgresql.org/docs/9.0/static/libpq-pgpass.html

Scheduling automatic backup of postgres database in windows

DOS Batch