I note that the thread title is somewhat of a misnomer---an active anti virus program is different from a passive scanner.
And what you want on your PC is something active that operates in real time and blocks incoming viruses and Trojans as they come in. And nukes them before they install. And a scanner is something that really operates after the fact--in that it examines your hard drive files and checks to see if you have a virus.---with the active anti-virus on your PC being able to scan also. Which is also a necessity---because there is always a time gap between the time a new virus is first released into the wild and the time it takes for various anti-virus companies to develop a signature file that actively identifies it. So if something slips past your active AV in such a zero day attack, a subsequent update may catch it in a later scan.
And one can read up on the detection rates of various active AV's, but the data is always dated and incomplete. The point being there is now so much malware in this class that no single product will even have ALL definitions in their signature files.
But there is one way to somewhat hedge your bet---especially given you can have one and only one active anti-virus installed at a given time. And that is to take one of a wide variety of free on line virus scans from places like trend micro,
Kaspersky, Panda, and others. It may take a full evening doing the definitions download on dial up, but its fairly fast on broadband. And because its a passive scan, it won't interfere with your active AV. And you can also use it as a selection tool, if you suspect your AV is not up to snuff. And if some on line scan finds something on your hard drive your active AV missed, you know it may be time for a change---and you could even have some AV with high ratings, and some less than stellar rated passive scan could find something the higher rated product missed.
But I will add the following note of caution---sometimes these free on line scans are a scam---not often with nationally recognized anti virus products but very risky when you move down to the spyware class---where there are pile of basically
rouge companies---and listed as rouge companies at places like spywarewarrior.com---that will either tell you you have a pile a infections when you don't--false positives---to scare you into buying their product--or even worse---some of these rouge programs will actually install malware on your PC.