Try booting up the system with the bare minimun. CPU, Video, and Memory. This will eliminate any possibility of failure with your other hardware (Etc.. modem, sound card, hdd, floppy, and so forth.) If your system boots with these components out, try booting with one component install at a time. This will help eleminate the hardware that is causing the problem.
If that does not work, boot the system up with the video card out. Your system board should give you a beep code indicating that there is no card install. If you get a beep code, put the card back in and remove the memory, and boot up without the memory(depending on you bios, it will give you different beep code for video and memory.) If you do not get a beep code for for your video or your memory, unseat and reseat the cpu. If that does not work, your cpu or system board might be bad.
This is just basic troubleshooting. Make sure that all your components are seated in properly.