I have a user who always seems to have an issue with his normal.dot file becoming corrupt. In the past I've deleted or renamed the normal.dot file, and reopened to create a new one. This seem to fix the problem. Yet, for some strange reason the file doesn't take with this user. I've deleted temp folders, defragged hardrive, and ran a virus scan just to make sure. As a last ditch effort i also reinstalled the app, which did nothing to help. I also checked to make sure he wasn't using the same word file with the same watermark added.
Does this user have a roaming profile? We've had odd issues at times where users will have part of a profile corrupt and that could cause something similar to this; we give them a new profile, completely separate from the one on the server, and it normally solves the problem. For awhile. Sometimes the profile becomes corrupt again if they are impatient or do things like flicking off the machine mid-save when logging off or on.
Have you run the Office/Word repair to have it check for unusual installation issues?
Is this user the only one that uses the computer, so you know if it's a user-centered issue vs. computer-centered issue?
Are you regenerating the file by just deleting it and having word recreate it or are you copying another user's template file over and replacing the "bad" file?
I'd also be tempted to take a "good" copy and a copy of his corrupt one and run an md5sum on them to see if there's a difference, and have him run through parts of his routine and re-compare to find out when the corruption is occurring. Compare right after you fix it, when he/she logs off and turns it back on (compare it using a network connection), logs in, etc. and see if/when the hash changes to narrow down if it's a Word issue, profile issue, etc.
Hope this helps to narrow it down for you.