To un-install (remove), use the add/remove programs utility found in the Windows control panel.
To re-install, run the .exe (the big file you downloaded), and when the "purchase screen" appears, ignore everything that is there, and just click on the REINSTALL button at lower right.
When you click on this button a browser window will open up, and you can then point the installer to where your key is located. The default location of this key is in a Flight One Software folder on your main hard drive.
Keep in mind that you will need to be connected to the internet when re-installing. As during this process your key is verified by Flight One.
All-in-all a pretty simple process, that probably took longer to explain than to do in real life!