I'd go ahead with the re-install.
Twice last year I had my install of FMP 10v3 on Windows XP go bad. In both cases, I had trouble running other applications at the same time though with different symptoms. (After a certain number of windows (or internet explorer tabs), were opened, opening a new window would fail to happen, it would open with windows objects missing or misplaced, or Filemaker would crash.)
Re-installing Filemaker and then upgrading to 10v3 fixed the problem.
Now I have arrived at my clients place, would like to remove FileMaker and reinstall FileMaker, but they don't have a copy of their original installation App.
I can download a Demo app, but how do I put their license number into the Demo App?
You can contact filemaker customer service and they can email you a download link for a copy of the installer once they've verified you are so entitled.