If the A/V is updated, do you set it to run by itself a few times a week? That helps keep on top of things, but also -Windows Defender is free and is a good suite of utilities for the automatic removal of Malware, Adware, etc. etc. Has many other useful little titbits too.
I got caught out last week, by a reported virus which wasn't going to move. Thing is that I knew the file that it was in and that was six months old. Norton's latest virus definitions had only just picked it up along with my heartrate. However, it just does illustrate, that you have to keep plugging away at the issue and that sometimes the truism that "No news is good news" might have to be modified a bit.
Conan