I received the error message "Invalid activation file"

First thing to check, do you have the latest version of Circle installed? It's always available here: http://www.futureaudioworkshop.com/circle/downloads/

If that doesn't work...

1. Copy your custom presets on to the desktop. The presets can be found in (Mac) "Macintosh HD/Library/Application Support/FAW/Circle-2/Presets" or (Windows) "C:\Program Files\FAW\Circle2\Presets".
   
   
2. Delete the Circle program folder: (Mac) "Macintosh HD/Library/Application Support/FAW" (Windows) "C:\Program Files\FAW\Circle2".
   
   
3. Reinstall the latest version of Circle, it's always available at www.futureaudioworkshop.com.
   
   
4. Reload Circle and you should be able to activate successfully!
   
   
5. Copy the custom presets that you place on the desktop back into the Circle program folder.