Avast Internet Security 8.0.1492 does not work. Windows 8.1 Preview pops up many processes' cancellations and if you 'OK' them, then the system crashes. I could not install any software, as Windows 8.1 wipes out all softwares, except personal data. I installed and uninstalled Avast and also reinstalled Windows 8.1 Preview without any antivirus. Now the OS is seemingly stable. The question is-Is Windows Defender good enough? The communication from the support team is that everything will be alright before the official release of Windows 8.1, which is October 26, 2013. My family has many licences.