Please do a full **clean** re-install of Steam and game:
- move any valued save games elsewhere
- "uninstall" in Steam-CK2
- exit the Steam app entirely
- manually delete both the Steam/SteamApps/common/CK2 AND Documents/Paradox/CK2 folders
In particular make sure there are no files or folders left under either the Steam CK2 or Documents CK2 locations, before installing again.
- delete everything in the Steam folder EXCEPT steam.exe and the SteamApps folder
- run steam.exe to rebuild all that
- re-install game in Steam, run a Steam Verify when done.
- re-add ck2game.exe to the AV whitelist
- start the game with no mods active and test
IF that doesn't help, DXDIAG is a program you run from a command prompt or the Windows start menu 'run' dialog box (or 'search programs' in Windows 7 or later). After running it will open a window and start collecting info with a progress bar in the lower-left corner. When it completes click the 'save all information' button and save it to a file then attach that file here.
Please attach your Documents/Paradox/CK2/settings.txt.
From your Documents/Paradox/CK2/logs/ folder, attach system.log .