Using Excel with Dual Monitors Windows XP, Windows 7 and Office 2010

I have dual monitors and a lot of times I want an excel file on each monitor.  Typically that can't happen because they are both contained inside of excel.  Here is how you can have it open two instances of excel.  Make sure Excel is closed.

Open Windows Explorer...  At the top select Tools -> Folder Options -> File Types

Scroll down to the excel file types.  The two types you'll need to change are xls and xlsx.

You will want to highlight that extention and click advance -> Select Open -> Click Edit

In the "Application used to perform action:" Box scroll to the end of the box and add "%1". You need the quotes.  Uncheck DDE and click ok.  Click ok and do the same for xls or xlsx.  Now open two excel docs and they will be in two instances!

Source: Link

UPDATED:

For Windows 7 and Office 2010:

NOTE:  ALWAYS BACK UP YOUR REGISTRY BEFORE MAKING CHANGES.  I am not responsible if you screw up your computer changing these settings.

Click the Windows Start button/orb -> Type Regedit

Navigate to HKEY_CLASSES_ROOT\Excel.Sheet.8\shell\Open\command

Double click the (Default) key and it should say: 

"C:\Program Files (x86)\Microsoft Office\Office14\EXCEL.EXE" /dde

Change to "C:\Program Files (x86)\Microsoft Office\Office14\EXCEL.EXE" /dde "%1"

Click Ok

Rename command to command2

Repeat the following for Excel.Sheet.12

Full credit goes to this guy for figuring it out since the previous way was not possible any more.  Link