Office 365 installed - Open Word 2010 by default

You'll need to reregister Word 2010 explicitly, using the full file path. By default (for 32-bit Office 2010), this will be C:\Program Files (x86)\Microsoft Office\OFFICE14\WINWORD.EXE. To do this:

1. Press Windows Key + R to open the Run box.
2. In the box, paste the full path to the Office 14 version of Winword.exe (as above).
3. Type quotes around the path and add the /r switch, resulting in
   "C:\Program Files (x86)\Microsoft Office\OFFICE14\WINWORD.EXE" /r
4. Press Enter. You'll get a message that Windows is reconfiguring.

If you will ever be opening Word 2013, you need to edit the Registry to keep the two programs from reconfiguring every time and grabbing back the file associations. See the "Hacking the Registry" section of the article at http://www.gmayor.com/Toolbars_in_word_2007.htm.

Reregistering Word should have no effect on Excel. You can reregister Excel similarly, however; in that case the command is "C:\Program Files (x86)\Microsoft Office\OFFICE14\EXCEL.EXE" /regserver.

If that doesn't do the trick, you'll probably be better off posting in the Excel forum.