Why Does Excel Change Cell Formats Automatically?

This is amazingly annoying, and apparently random: I have several workbooks that I use quite often, and I spend most of my time within one sheet. I update values, mainly. I never change formatting because I have already established that when I set the document up.

The problem is that, on occasion, when I open the file, the cell formats are ALL changed to TIME. Cells that were dollar amounts are now time. Cells that were addresses are time. Cells that were dates are time. Cells that were blank are time. The entire document has magically shifted every cell to time.

Aside from the effort needed to correct all the cells, the formulas break and search doesn't work.

Am I the only one with this?

The files reside on a local network and could be accessed by someone else, however they aren't because I know and trust that guy.
Windows 7 Pro SP1 (legit, purchased)
Office Standard 2010 (v:14.0.7153.5000) 64-bit (legit, purchased)

Interesting... Didn't know any of that. Turns out there are hundreds of "Normal" styles, but I found what appeared to be the root version and edited it. It was indeed set to time.

UPDATE:

Excel continues to randomly change the format of the cells –sheet-wide– to time. Editing the normal cell style does NOT revert all the cells from time back to general, so I have to continue reformatting my doc. Ultimately, I think I will have to move all my data to a fresh doc.