I installed 5 over 4.8, and still see the "anti-virus" category in the event viewer too even though there's nothing in it. I guess which way you installed makes an unexpected difference, for whatever reason.
Slightly off-topic, but closely related ... I've also got the Internet Explorer category in my event viewer, though I can't remember ever seeing any events there. (I've got IE-8, but only use that for MS Updates, Firefox for everything else.) Is that probably an analogous situation, a carry-over from some older IE version that's no longer around or active?