Users are reporting compatibility issues with the first Windows 8 update
Even though Symantec has already announced that its security products are fully compatible with Windows 8.1, it turns out that most anti-virus apps on the market do not work on the first Windows 8 update.Several users have confirmed on Microsoft's support forums that anti-virus software doesn't launch on Windows 8.1, including apps such as Norton 360, Avast Anti-Virus, and Micro Titanium Maximum Security Suite.
At this point, the built-in Windows Defender seems to be the only choice for users who wish to give Windows 8.1 Preview a try. Windows Defender is pre-installed on all Windows 8.1 machines and given that it's included in the operating system, the CPU and memory footprint should be minimal.
Security companies are most likely working to make their apps compatible with Windows 8.1 Preview as we speak, so expect a new wave of announcements on this particular topic anytime soon.