In June Mikhail asked the same question but the answer was how to do the opposite (make multiple instances the default). I am trying to get to an answer to Mikhail's question which I rephrase as :-
I have same problem with 64 byte Windows 7 and Excel 2007. Excel always used to open in a single instance n o matter how/where I opened the sheets. Because of this I could always copy and paste, etc with full formatting, formulas, etc. Suddenly, Excel switched to opening everything in fresh, separate, multiple instances and destroyed the basic cut and paste options.
Wasn't the original question how to go back to everything in a single instance ? I have been searching for the answer to that question (rather than the opposite).