No point in trying 3rd party software. XP seems very confused as to whether the files exist or not. Strange is not the word for it... more like bizarre.
The most unusual thing is that what is backed up automatically on a daily basis (I use Back2Zip) is different from what is showing in the folder on C: drive. The main folder on C: shows only the subfolders containing the files that won't open. The backup copies the main folder and it's contents as if I had never moved them, but the subfolders are empty. So, the backup thinks I never moved the revised copies into the subfolders, hence I have copies of the originals, but not the latest versions. It's kind of hard to explain without attaching six screen shots.
Bottom line is I've already wasted too much time trying to figure this out. I will just have to start over again from the originals. I am going to try to move the files into the subfolders, only this time I will save in .doc format and see what happens.
There appears to be something freaky about how XP recognizes files with an .odt extension. Perhaps Microsoft is trying to squelch the (free) competition.