PC startup problem

Just wanted to find out what I should do to rectify the problem of a PC that seems to be booting up fine (although the screen does not come on but it sounds the way it should), the lights for the CD rom and CD burner blink but when the floppy drive light comes on, it continues to blink and the computer beeps once every few seconds all the while the floppy drive light comes on and it goes no further than this. I have tried replacing the memory, the floppy drive, the hard drive and the video card with no luck, it currently has on board video and I used a PCI video card but still it keeps beeping. What is the problem-can someone tell me.