Bit of history would be helpful - this could be caused by hardware/software or leftover of previous issue...
Have you checked the hard drive (scandisk, drive manufacturer's diagnostic utility - go to their website)?
The RAM? (eg, run memtest86 - google to find, or use known good, or just one stick at a time if > 1).
Disconnect unnecessary to boot hardware (eg, CD/DVDs, sound, network, modem cards etc) - see if it will boot - put back one at a time if it does.