I use nod32 av v8 in windows 8.1 its fine.
When I tried v9 (admittedly during beta), its HIPS was broken (rules got corrupt) and I hate the new UI which is slow and too big.
However according to marcos on the eset forums they plan to roll out an update for v8 users to update to v9
, after which I assume v8 may become unsupported.