After checking the memory and video card, i next look at the power supply. Speedfan is a stand alone program That not only reads your temps and fan speeds it will also read the voltages of the PS. If you do get it to boot install speed fan and see if the voltages are low. I found a puter i was working on crashed after 15 minutes every time i turned it on. I found it had a low 12 volt line( 11.73 volts). Switching to a PS that had over 12 volts stopped the lockups. Once you have checked the PS, it is time to disconnect all the cdrom drives, sound cards, floppy drive, etc. I usually try booting with the MB on a box with only a stick of memory, video card, processor and hooked to a power supply. You should get to the bios this way(clicking the proper key to get to the bios). If it still crashes, either the MB or processor is bad or going bad.