If you’ve a Windows 8 computer that has Avast installed, before upgrading to Windows 8.1 you’ve to make sure you’re using latest version of Avast i.e 2014. Because Avast 2014 has got Windows 8.1 certification and that’s why it works well without any issues on  Microsoft’s latest Operating System. But older versions of Avast like Avast 8 and prior versions haven’t got this certification, hence they don’t work and in fact any such version will be disabled after Windows 8.1 upgrade.

So to avoid this, first update Avast version to 2014 via program updater in Windows 8 and restart your computer then install Windows 8.1.

"Avast! 2014 has Windows 8.1 certification which ensures that it meets compatibility standards and performs well on the Windows platform. Older versions of avast! Antivirus do not carry that certification, so if you update to Windows 8.1, it will disable avast! and you will be unprotected." Avast said in its blog.