i have a 2004 computer with no operating system(used to be an xp pro). i tried using the windows xp operating disk and switching the bios to boot from CD but the DOS like text says "NON OPERATIVE SYSTEM
INSTALL BOOT DISK". or something similar
Your optical drive may not be working properly. As a first step, get a cleaning disk and see if you can remove any dust inside.
Within BIOS Setup, are both the optical disk and the hard drive units "seen"? Are both Enabled? Since you are trying to install Win XP, IF your HDD is SATA, make sure its SATA Port Mode is set to IDE (or PATA) Emulation, and NOT to SATA or AHCI. Check that the Boot Priority Sequence is set to optical drive first, hard drive second, and NO other options.
IF on the other hand your drives are both IDE, ensure the jumpers on each are set correctly. The HDD should be the Master device; connect it to the black END connector of the ribbon cable. Then set the optical unit's jumpers to Slave and plug into it the gray middle connector. THEN check the BIOS settings as above, except that the SATA Port Mode setting does not matter if you are using only IDE drives.