I have FO3:GOTY. I'm running GECK 1.5. My OS is Windows 8.1. GECK will open just fine, I can load multiple masters just fine, I can browse items just fine (although I'm coming from 400+ hours in the Skyrim Creation Kit and GECK doesn't handle quite the same way, despite looking exactly the same), but as soon as I try to load a cell (interior or exterior), it gets halfway through loading the objects and/or terrain and then I get the message that it's stopped working and to close the program. Even if I just load the main .esm and pick a random vanilla cell, it does this. It's super frustrating, because all I want to do is build things, and clearly I can't when it's doing this. I hope someone can help.

Update: Setting compatibility on Vista Service Pack 1 is now allowing me to load and switch between interior cells, but exterior cells still crash the GECK at about the three-quarters loading point.

The GECK can be very tempermental, it took a while to get mine to even open up. Can you run falllout 3 ok with no crashes? I would suggest regardless if not to:

Open up the fallout.ini file in: My Documents\My Games\Fallout3

Find the line:bUseThreadedAI=0change it to:bUseThreadedAI=1Add another line after it and insert:iNumHWThreads=2

This will limit the game to 2 cores and prevent the engine bug from causing the game to freeze. Also install the http://www.nexusmods.com/fallout3/mods/19122/? as this fixes some errors in the geck.

Also make sure you have the 1.5 update of the GECK http://download.zenimax.com/fallout/3/geck/Fallout3_GECK_1.5_Update.exe