Go to http://support.microsoft.com/kb/211494 and it will give a very clear explanation of how to make it so Excel opens the file. I searched on: "Excel 2007 'There was a problem sending the command to the program'" since that was the error message.
Open the Excel application. Here are Microsoft's instructions from that link - very simple to follow!
Microsoft Excel 2010
Click the File tab, and then click Options.
Click Advanced, and then click to clear the Ignore other applications that use Dynamic Data Exchange (DDE) check box in the General area.
Click OK.
Microsoft Office Excel 2007
Click the Microsoft Office Button, and then click Excel Options.
Click Advanced, and then click to clear the Ignore other applications that use Dynamic Data Exchange (DDE) check box in the General area.
Click OK.
Microsoft Office Excel 2003 or earlier versions of Excel
Click Options, on the Tools menu.
Click the General tab.
Click to clear the Ignore other applications that use Dynamic Data Exchange (DDE) check box, and then click OK.
Even if the option Ignore other aplications that use Dynamic Data Exchange (DDE) is already clear (false), click to enable and then click to disable.
After you do this, you should be able to open workbooks by double-clicking them in Windows Explorer.
Best Answer
Sound like your default file association has messed up. I could go into a long lengthy explaination of how to fix, or let Microsoft do that for me.
http://support.microsoft.com/kb/307859
the fix itself is easy, its just buried a few menus deep.