Hi,
In our latest release we added expanded support for Windows 8.1. However, we still do a hardware check of all machines to check graphic card and other compatibility before we allow the GrimeFighter offer to be shown. That means, if you don't see GrimeFighter in the Avast user interface your PC is not supported. Windows 8 and 8.1 use UEFI boot mechanism instead of BIOS in many cases.
The latest release added support for UEFI as well as SecureBoot, which is also quite prevalent in Win8 and 8.1 machines.
Hope this helps,
Davidek