Well, when you update FPS Creator and you had the full version, it will more than likely go back to the original version (Free version). Well, I tested this on 3 of my laptops (Vista, 7 and 8) and I figured a way to fix.
I'll put this step-by-step
1. Go to your FPS Creator file location (C:\Program Files (86x)\The Game Creators\FPS Creator Free)
1a. You can simply do this by right clicking the shortcut and click "Open file location"
2. Scroll down until you see userdetails.ini
3. Copy that and place it on your desktop
4. Right click it and open it with WordPad, not Notepad.
5. Put your serial code that you recieved on the YourSerialCode= line. Example, YourSerialCode=YourSerialCodeGoesHere.
6. Save it and drag it to your FPS Creator Free file location.
7. Open up your game and go to File > Build Game and you should be able to use it.
I hope this helped some of the newbies.
The advanced game creator + Good listener.
Taking school for game creating and using FPS Creator for starters.