A Fix for Far Cry 4 won't launch in Windows 8.1
For me the problem wan't USB, I only have keybd and mouse plugged in and unplugging either/both of those made no difference,
for me it was the Early Launch Anti-Malware Protection that Windows 8.1 runs on Startup.
I tested this thoroughly in different combinations and sequences and when it's Enabled FC4 will not launch, when it's Disabled it launches every time.
To disable this, from the Start menu (lower left corner of screen)
1. HOLD THE SHIFT KEY DOWN while selecting "Shut Down or Sign Out/Restart"
This restart brings up some menu screens, select...
2. Troubleshoot
3. Advanced Options
4. Startup Settings
5. Restart
6. 8) Disable Early Launch Anti-Malware Protection
Windows will now appear to load and launch normally, minus whatever anti-malware processes that were being run before.
Note that your anti-virus software now may not be running.
Go ahead and re-enable your av software manually if you want, or experiment and try to launch the game first. Either way, for me whether I manually re-enable Windows Defender (the only anti-virus I use) or not, Far Cry 4 still runs every time because it's not the anti-virus program that's causing the problem, it's the Windows Early Launch Anti-Malware thing.
To re-activate Early Launch Anti-Malware Protection, simply Restart your PC like you normally would.
DISCLAIMER: The only minor (in my opinion) down side to disabling the Early Launch Anti-Malware is if some virus is trying to get low-level access to your machine and bypass your anti-virus software. Again, in my opinion, this is so E-x-t-r-e-m-e-l-y Rare that if you disable it for a few hours to play a game, then reboot normally, it'll be fine.