pg_dump exists without any message when running from windows task scheduler

In some servers 8.4 pg_dump exits without any message if running from Task
Scheduler.

To explore this, I created scheduler task containing line

cmd

After that I can type manually commands:

C:\MyApp\pg_dump>dir /w
..
[.]              [..]             comerr32.dll
gssapi32.dll     k5sprt32.dll     krb5_32.dll      libeay32.dll
libiconv-2.dll   libintl-8.dll    libpq.dll        msvcp71.dll
pg_dump.exe      pg_dumpall.exe   pg_restore.exe   ssleay32.dll
zlib1.dll

C:\MyApp\pg_dump>pg_dump

C:\MyApp\pg_dump>pg_restore

C:\MyApp\pg_dump>pg_dumpall

As shown, all commands exit without any message. This also happens if
command line parameters are passed. So it is not possible to use Task
Scheduler
to create backups.
If same commands are executed from same user account from command prompt
directly, they run OK.

How to create automatic backups in Windows ?

backups are created in server running Windows XP Prof SP3
PostgreSql 8.4 server is installed to C: drive and required files are copied
to separate pg_dump directory

Andrus.