Mac Forums

I recently bought an iMac from someone that was running off an external hard drive as the primary, as there is something wrong with the internal. Yesterday I decided that I would wipe my computer and do a fresh installation of OS X. After I deleted everything I tried to install OS X but it kept telling me that the software couldn't be installed on the computer. I think that I may have accidentally deleted what was connecting the external hard drive as the primary?? I don't know that much about computers. My question is, how in the world do I fix my computer and install Mac OS X on there if I can't even get into the computer?
The only problem with that, Joe, is that in Disk Utility it's not detecting that the external hard drive is even connected.