No apps work anymore (Win 8.1)

I can't open any app anymore, except for the store, but I can't install any apps anymore. I always get error 0x8007064a when I try to install apps. How can I fix this problem and what is it caused by?

In the Run dialog (+R) type WSReset.exe and hit Enter. Then reboot and try running/updating your apps again.

If that doesn't work then go to Control Panel\All Control Panel Items\Troubleshooting and click on Fix problems with Windows Update and then Try troubleshooting as an administrator:

When the troubleshooter's done its job reboot and try running/updating your apps again.