I have had this several times now, someone sends me an excel file and I cannot open it.
First i thought this was a compatibility problem as I tried to open xlsx files with the compatibility pack, but now I asked him to send it as a csv file just to be safe and the problem still occurs.

When I open the csv files with notepad it looks like a lot of gibberish, the only hint that I found was that the file starts with:
<## NASCA DRM FILE - VER1.00 ##>

I really hope that someone can help me because I would feel really bad if I had to ask this person to send the files again, and I have no reason to believe it will be succesfull.