Perform the following steps to have Excel open each file in a new window:

My Computer
Tools
Folder Options
File Types
Choose XLS
Go to Advanced

Uncheck "browse in same window" in advanced window.

Then highlight Open
Edit

Make sure in the Action box it says &Open

Make sure in the application used to perform action it says:
"C:\Program Files\Microsoft Office\OFFICE11\EXCEL.EXE" "%1"

Check the box next to use DDE 

Remove anything that is in DDE Message box and DDE Application Not Running box.

Make sure the application box says: EXCEL

And in the Topic box it says: System