system
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The problem is with Excel 2013. The computers are running Windows 7 Pro. One PC is running Office 2013 downloaded from the web. The other is running Office 2013 pre-installed with purchase of PC.
I followed the instructions offered by MS in the link in my first comment. I de-activated animations and deleted Avast. After restarting my computer (the one with the pre-installed version), I had a single crash, but have not had one since. This appears to be a fix. Since then, I have downloaded and installed AVG anti-virus and have had no problems.
I will follow the same procedure with the other PC when I get a chance.