zimon, Take it down to bare minimum. You want to go as low as what will work. Strip it of it's cd-rw/dvd/floppy. Take out all but ONE stick of memory. If you have on board sound and a sound card, take the sound card out and use the on board sound. When you have it down to literally bare bones, see if it will work and then start adding back ONE item at a time. When you get to the point where something *craps* it out, you've found the culprit. You might also what to check the power supply. Make sure you use a generic mouse and keyboard, not a wireless or optical.
Take notes every step of the way and post back your results. Liz