Can't open Word by clicking on documents

Problem with command-line switches: Can't open Word by clicking on documents

Using Windows 7 64-bit.
When I click a Word document in Explorer or on the Start menu, I get the error "There was a problem sending the command to the program". I'm using Word 2003, but I don't get this problem with Excel 2003.
I know that when I try to open a document this way, the program uses the command switches "/n" -- which means "open a new instance of Word", -- and "/dde" -- which I don't know what it means. I do know that I don't want the /n switch.
1) How can I remove the /n switch? Remember, I'm not clicking on a shortcut, but on a document icon. I know how to assign the "Open With" command, but I don't see any options about changing command-line switches.
2) Is the /dde switch causing my problem? How do I remove it? 

Compatibility mode doesn't seem to make a difference.
This doesn't happen with Excel 2003, or with any version of Office 2007. I'm sure this has something to do with how I associated .doc files to Word 2003: I'm guessing that it added the switches, and they're causing the problem. I wish I knew how to change the switches.