Enter superuser mode-problem

[Pushparaj Shetty]

have installed XUbuntu Desktop 6.10.

I have   problem in Ubuntu. I cannot enter super user mode(in
spite of  giving correct passwd).
$su
password:

Authentication failed.

But it same passwd is accepted in case of a other GUI based
application( like change date &time)
Please help

Thanks
Pushparaj

-- 
Department of MACS
National Institute of Technology Karnataka, Surathkal
PO Srinivasnagar
Mangalore-575025