I got my answer.
I want to thank Yeroen for his reply to my question. 🙂 However, I had already given up on getting a response from the forum by that time, so I had decided to totally reinstall the system from the DVD. That failed as well. 🙁
Then I took the computer to a technician, who did some testing and informed me that my hard drive had failed. I had to get a new hard drive. 😱
Fortunately, I had done a backup immediately before attempting to upgrade the system, and it was during that attempted upgrade that the problem developed. Therefore, I lost absolutely nothing except the money for the new hard drive. 🙂 🙂 🙂