I've been struggling a couple of days with this problem and found the fix so I thought I'd post it here and hopefully save someone the same trouble I had. I use Win2k and Word 2k and all the sudden yesterday whenever I went to open a Word document by double-clicking on it I got the message: The document name or path is not valid. I tried everything and it was happening with every Word document I tried to double-click on. The only way I could open the document was in Word Pad which caused me to lose all my formatting. I never figured out what caused the problem, but here's what I did to fix it:

First I did start/run and entered the following command: winword.exe /r (you may need to enter the entire path like this: C:Program FilesMicrosoft OfficeOfficeWINWORD.EXE /r)
After you've run that, then in Windows, r-click on the start button and click "Explore." Go to tools/folder options and the "File Types" tab.
Under Registered File Types, select "Microsoft Word document" then click "Advanced"
In the Edit File Type box under Actions, select "Open" and then click "Edit"
In the Applicaiton Used to Perform Action path, enter the path to Winword. After winword.exe there should be a space, then /n then "%1" WITH QUOTES. Here's an example:

"C:Program FilesMicrosoft OfficeOfficeWINWORD.EXE" /n "%1"

If the /n "%1" is missing at the end, then add it. If the quotes are missing, add them.
Close all windows.

This fixed the problem for me without a reboot.

Like I said, hope it helps someone somewhere someday.