I don't have another guess. I've never seen excel do anything like this.
You are sure you opened in Safe mode so that all macros were disabled?
(You do see "safe mode" in excel's title bar, right?)
One more thing to try (that may not help)...
Open excel in Safe mode
File|Open and click on your workbook
but instead of clicking on the open button, click on that dropdown arrow on the
Open button. Chose ReadOnly.
What happens then?
