Reversing jack-in-the-box icons that replace old excel file icons

Today I noticed that Excel file icons on my desktop have been replaced with a jack-in-the-box icon. It looks like this:

This seems to happen only to Excel files last modified prior to 1993 (although I have not checked all files to determine if this is always true). In the information panel (command-I) for the file, under "open with" it shows "microsoft excel (110725)" but with a "do-not-enter" symbol superposed. On the "open with:" pull down menu (still in the information panel) it also lists "microsoft excel 14.3.4" two times, along with "print preview" as possible choices.

If I double click on the file, it opens normally (I looked long and hard to see if this symptom resembled any viruses before doing that). But it does not change the icon back to normal unless I resave it as an xlsx document. If on the information panel, I change the "open with" choice to microsoft excel 14.3.4 and add the .xls extension to the file name (still in the info panel), the icon changes to the normal Excel icon. It does not change the icon on the desktop unless I do both. Occasionally, it will change the icon on the information panel, but not change the icon on the desktop.

In searching through the archives I found a discussion that suggests this icon is residual from the ResEdit program. I have no idea if that is relevant to my case (in the archive, it had to do with icons for Word documents).

I would appreciate thoughts on why this may have suddenly occurred, and if there is a way to reverse it, short of going through each of hundreds of Excel files, opening the Information panel, and manually making the change.

I did not recently update my OS or upgrade Office. I am running OS X v. 10.8.3 on a 17 inch MacBook Pro with 2.2 GHz Intel Core i7. The Excel version is 14.3.4 (130416). I have Sophos Anti-virus software running.

Thank you.