HD's not being recognized?

I'm running an old Compac desktop. Windows 2000, 130 Ram, 6Gigs, 400mhtz, about as slow as a turtle on its back running from a forest fire...

I changed HD's, loaded a new one with an OS, and after I removed the second HD, and replaced the original HD, the machine won't boot. Then I installed the second, and it still won't boot. Plus, when It starts, it says "keyboard error", and I've tried a few different keyboards, with no success.

I'm guessing the problem is a setting in Bios, but everything I've tried in Bios was unsuccessful, so I pulled the battery, and that did nothing too. So I "pulled my hair out", and still nothing. I guess you know the feeling.
Um, I really don't have a clue what to try next. A new tower dropping into my lap would be nice, but prayer hasn't helped neither, so it would seem I haven't even got a prayer.

What happens in a computer when you change hard drives?
Can it damage something in a tower?