womanclone

I bought a used compaq and started it up but there was no display showing at all. I know this display works because I disconnected it from a second computer that was working fine with the same monitor. First I double checked the cord to computer connection and everything seemed ok there. Since the video card is an onboard type I decided to take a working video card from my parts box and plug it into one of the open slots on the motherboard and still nothing. Thinking well there is the possibility that this card could be defective I unplugged it and put another card in and still nothing. Next I moved on to the hard drive thinking that the hard drive might have been wiped out or had a virus or something? I put a hard drive from an operating computer in and that wasn't the problem either. One thing that I find strange is the monitor screen is totally blank and not even the bios part comes up so could there be a problem in the bios? Does the battery on the motherboard needs replaced?