Now that I have been upgraded to Office 2013 I have problems with opening more than one file at a time.
I use 3 monitors for working on multiple spreadsheets. Usually I display the files across two monitors and use the third for VBA window, or reading email, or other work. I often open multiple files from the Windows Explorer.
If I open one file it will open to the size and location I closed the last Excel file, but if I open multiple files only the first opens correctly the remaining files always open on the far right monitor (correctly sized) with half of each new window beyond the edge of the viewable desktop.