Excel (xls) files spontaneously converting to text files
 

On several occastions I have had an Excel (xls) file spontaneously convert to
a text file. When I open a file I had been working on, the "Text Import
Wizard" pops up stating that the data is delimited. Once this occurs, the
file is lost, because the data is shown in a "garbled" format that means
nothing to me.

At times, I do work with a totally different file that I downloaded from the
internet, in a csv format. Could this be corrupting or converting other
files?