Remove your current fpsc installation (maybe cleaning your registry after you've uninstalled fpsc might be a good idea, you can use ccleaner for this task). Then follow these steps:
1) Install fpsc again with your name and serial.
2) Download the v1.20 BETA 15 update from here
http://files.thegamecreators.com/betafiles/FPS_Creator_V120_BETA15.zip.
3) Download the Community Mod from here
https://sourceforge.net/projects/fpsccommunity/files/latest/download.
4) Install the update v1.20 you have downloaded.
5) Launch fpsc.
If it sits there, telling you "checking files for you", and then suddenly crashes you have to make sure that two files, msvcr71.dll and msvcp71.dll, are present inside the main fpsc folder (the SAME where FPSC-Game.exe and FPSC-MapEditor.exe are): you can find the two .dll files inside the Community Mod archive, extract and place them inside the folder I've told you.
Now launch again fpsc. If it crashes again, make sure that you have UAC turned off (Windows control panel, turn the UAC settings slider down completely) and that you're executing fpsc with admin premissions (right click on FPS Creator.exe and choose "run as administrator"). If fpsc now launches, follow the next step.
6) If fpsc tells you it's a trial version just do this:
Close fpsc and open up the folder where it's installed, the same where you've placed the two .dll files before. Here you should find a file called userdetails.ini: open it with notepad. You should see two lines: one is YourName= , the other one is YourSerialCode= . If they are blank after the = symbol, then that's why your fpsc thinks it's a trial. Just write your name after the YourName= line and your seral after YourSerialCode= . Now save the file and close it. Run again fpsc, now it should be recognized as full version.
7) If you want to install the Community Mod, you just have to copy all the files from the mod zip folder to your fpsc installation folder (choose yes if it asks you to overwrite some files): just make a backup of the official FPSC-Game.exe, by changing its extension to something like FPSC-Game.exe.backup...and you're done!!
8) Run fpsc: now the mod will be running.
Hope this will work for you!!
Good luck!