1 always turns to 100 PLEASE HELP!!!!!!

I work as a troubleshooter at a small college where I live, and I am really stumped right now. One of our faculty members informed me that whenever he typed a 1 into a cell it automatically became 100. He is using EXCEL 2000 with Windows 98. This faculty members teaches an EXCEL class, so I knew it wasn't a simple formatting issue. When you type in 10, or 100 etc. it works fine. The problem only exists when you type in a 1. Another strange thing is that this problem didn't always exists. We are actually able to open old worksheets that were created before the problem started, and they work fine. I can even copy a 1 from and old worksheet and paste it into a new worksheet and it works fine. Yet it will not allow me to type a 1 staright into a new worksheet without changing it to 100.

Any help or suggestion will be greatly appreciated.

Sincerely

Robert H. Shanklin