Steam keeps validating
To fix this problem either wait for anti virus update that fixes the problem, or add the problematic file to your anti virus exceptions.
After that, validate the game on Steam to redownload the missing file. While it shows 0% all of the time, it actually downloads your subscribed Steam Workshop mods.
Please make sure that you have logged into the correct Steam account that you bought the game(s) with.
Please make sure that you have selected the full list of your games in Steam Library, not a list of currently installed games: In case of a gift, the game may have appeared in your Steam Inventory instead, in which case you must navigate to your Steam Inventory ( Hover over your name in Steam and select Inventory ) and redeem the game from there.
If you are having issues with your steam bought version of King's Bounty Crossworlds, you may want to try this.
Thus it can go into a continuous loop between the game and it's expansion, and unless stopped by the user, or steam or whomever, will just continually invalidate what steam downloads for those files that failed validation on KBCR during it's validation of KBAP and visa versa. Download fresh copy of KBCR, and then validate it, it may or may not find failed files, depending if steam ran validation for KBAP after you installed KBCR.
ref=3134-TIAL-4638 Certain Steam issues may be corrected by renaming or deleting the Client file and restarting Steam.
Here's an article from Steam Support describing how to do so: https://support.steampowered.com/kb_article.php?
If it found failed files, go ahead and download them.
Now as long as you keep the auto updating turned off for KBAP, and don't manually try to validate the files for KBAP, you should be good to go.