I used to have Word 2000. I upgraded to Office 2007, but then I wasn't able to open any word documents, no matter if they were old or new. I could open MS Word and then open the file, but I wasn't able to double click on the file to open that particular document. I checked file associations - Word wasn't even an option (though clearly it was there and I could navigate to it). I had tried all sorts of hotfixes and updates and converters and other sorts of potential fixes. Uninstall/reinstall didn't work. Running the program repair utility (via control panel) also didn't work. I read that it could be a registry file, but wasn't able to figure out which one and the ones I found online for other issues did nothing for this.
I did finally get the fix after calling Microsoft and having an unusually easy and quick chat with them. It WAS a registry file for the file associations for .doc and .docx files. If you navigate through your registry and go to HKEY_CLASSES_ROOT - you will see a bunch of file extensions. I deleted the .doc and .docx entries. At this point, you can do 2 things. 1) run the Office repair utility through the control panel in "Programs and Features" (if you're on Vista, otherwise it should be the traditional Add/Remove Programs section); or 2) copy those registry keys from any other machine that's working properly with the version of Word/Office you're using.
Anyway - that's what worked for me if this sounds like the issue you're having.