When opening multiple excel files they used to all open in separate versions of Excel which was great because then I could easily Alt-Tab between them. Now all of a sudden when ever I open new excel files by double clicking the name they open in the same Excel program but as a separate window. This is a real pain because it's hard to cycle through them and they keep minimizing whenever I maximize another one.

I haven't changed any settings that I know of, is there a way to get each file to open separately again?