So the game used to run on this same computer OK until 2 days ago, is that right?
If so: What change or update was there to Windows or this computer between the last time the game run okay, and the first time this problem happened?
Do EU4 and Stellaris still work OK?
What antivirus app do you use? Add steam.exe and ck2game.exe to its Exceptions list.
If none of that helps, we may as well try a full **clean** re-install:
- move any valued save games elsewhere
- "uninstall" in Steam-CK2
- manually delete both the Steam/SteamApps/common/CK2 AND Documents/Paradox/CK2 folders
- re-install game in Steam, run a Steam Verify when done.
- start the game with no mods active and test
In particular make sure there are no files or folders left under either the Steam CK2 or Documents CK2 locations, before installing again.
If still no good:
Please attach your Documents/Paradox/CK2/settings.txt.
From your Documents/Paradox/CK2/logs/ folder, attach system.log .