Booting problems

My system won't boot using GRUB
Every time of booting I get the error : ERROR LOADING OPERATION SYSTEM
The try to change the harddrive mode to LBA in the BIOS don't help.

But when I instert a WinXP install CD bevor booting, and wait this few 
seconds don't starting from CD by pressing any key Grub starts and I can 
make my choise to boot Windows or Ubuntu!?!?!?

So, what's that? Have you any idea to solve this problem?

Greetings from Germany
 Ralph



Here now my sytem:

Board    :    MSI K7N2G nForce2 Chipset
CPU      :   AMD AthlonXP 3000+
RAM     :   512MB DDR Infineon
Floppy
hda        :   Maxtor
                                       20GB
               Cyl.           39.813
               Heads              16
               Precomp            0
               Land.Z.     39.812
               Sect.                 63

hdb         :  Samsung
                                      160GB
               Cyl.            65.535
               Heads               16
               Precomp            0
               Land.Z.      65.534
               Sect.               255

Sound   :   SoundBlaster Live 5.1
Net.      :   OnBoard
Graphic :   ASUS ATI Radeon 9550, 128MB Ram
DVD     :      Toshiba SD-M1712
                   Plextor PX-708A