Have you tried taking any of the workbooks to another computer with Excel on
it and seeing how they look on that computer?
I agree with Gord Dibben that there is a potential that some malware has
done this, although I haven't heard of one just picking on the contents of an
Excel workbook - but stranger things have been done. Because of this risk,
you may want to be certain that the other machine has a strong, updated
anti-virus application running on it.
There are several sites that offer online scanning of both your computer and
individual files, one that I trust is at Kaspersky Labs:
http://www.kaspersky.com/virusscanner