Win 8.1 will be supported for some time yet (I haven't got a date), currently win7 is still supported and XP Program updates ended with 18.8.xxxx. With Win 8.1 you should still be able to get the latest versions provided the CPU supports SSE3 (which I would guess it does, this is what I believe killed XP support).
I don't believe there is an Engine version as such there is the Program Version and Build number and a UI Version. The virus definitions is a date based identification, year/month/day (22052508, the 8th VPS update today) and the updates that day, check the images I posted
I'm still on Avast 22.2.xxxx, my details are in my signature below my posts.
Avast are unlikely to be able to check individually with out information, which I believe would be tough with such an old version and would they fix it for such an old version, I just don't know as an Avast User.
Ordinarily a system restart would be first, a Repair the next port of call followed by a clean reinstall. Which depends on you and sticking with old versions.