nikkisboricua 0 Light Poster

I have a computer that won't read any disks on startup. It was running before, but I decided to reformat the hard drive and start from scratch. I tried installing windows 2000 pro and it read it on startup, but encountered an error mid-installation. I guess the CD was scratched. Well, after that, it won't read CD's on boot anymore. The BIOS are the same. Boot sequence=CD, floppy, HD. Well, I decided to put XP instead. It won't read the XP CD. So I took out the HD and placed it in the other as the only HD and installed XP just fine. XP ran, I set it up, and even worked on the desktop. Well, I transferred the HD back to the original computer from which it came, but after it boots the BIOS, the screen says 'no signal' and it restarts itself. It doesn't show the Windows XP loading screen or anything. When it restarts, it registers and says 'xp failed to load.. blah blah blah. safe mode, etc." I'm stuck at this point. Any ideas?