So this used to work on this same machine, with no change made to the machine (such as a Windows re-install)?
Please try a full clean re-install:
- move any valued save games elsewhere
- "delete local content" in Steam-CK2
- manually delete all files and folders left under Steam/CK2 AND Documents/Paradox/CK2
- re-install game in Steam, run a Steam Verify when done.