Word 2007 files defaulting to Outlook

*.docx files want to open with Outlook. I have Windows 7 OS and MS Office 2007 -- except I'm using 2003 Outlook because that's the only version I own. Everything worked fine. Then last week I sent an attachment to myself from my work email and opened it at home. When I saved it there was a small Outlook icon at the bottom right corner. Now I have to right click, use "open with" to get to Word. Error message reads: "Cannot start Microsoft Office Outlook. The command line argument is not valid. Verify the switch you are using."
I don't know how to verify the switch. I have attempted to fix this in file associations. Outlook appears as default and attempting to change the program does nothing. I can change any other file association, not this one.
What happened? How do I fix this?
Thank you in advance for any and all suggestions.