1. Close down any existing Serenji.
2. Close down Serenji Sentry (the blue GJ icon) from the System Tray in the bottom right corner, by right-clicking on it and choosing "Exit".
3. Find Serenji on your Start button's menu or equivalent, right-click on Serenji and choose "Run as administrator".
4. Enter the Activation Key in Help\Register. OK the dialog.
5. Close Serenji down, and also close down Serenji Sentry again.
6. Launch Serenji normally (i.e. not using "Run as administrator").
7. Go to Help\Register and confirm that the Activation Key is still present there. If it isn't, email support@georgejames.com for advice. Assuming it is, you should be all set to use Serenji.