Computer Runs, Monitor Doesn't Show Anything

So I've been working on fixing some of the old computers that my mom had laying around in her basement, just to get some of the old files off of them, and ran into a problem I can't quite figure out.

When I put in one harddrive, a harddrive that has been wiped clean of everything including an OS, the monitor works and everything is peachy keen, with the computer telling me that I need to install an OS if I wish to use it.

Here's my problem, I switch to a harddrive that I know has an OS on it, but when I do, the monitor shows nothing, though I can hear the beast starting up.

I'm quite perplexed on how to fix it. Everything else checked out fine, its just the monitor issue that does me in. I've tried both the videocard and the onboard for this second HD, except neither works.

They are both non-sata HDs. The Sata one is the next one I need to fix.

Nothing on screen on this computer, fires up, I know that its the original monitor that came with the computer.. You can hear it running and tell that it is going through everything it needs to do, just doesn't want to show the picture, I tried a different cable to make sure too.

GBoot problems:
http://www.tomshardware.co.uk/foru [...] _13_0.html