Kaspersky 5.0/NOD32 are at the top. I'd say NOD was, until Kaspersky released version 5.0, which now has a heuristic module built in. It puts it back on par with NOD with respect to ITW/new viruses. (means it doesn't require a virus pattern to identify suspicious files).
What sets it apart is that Kaspersky does a much better job identifying trojans than NOD does. Though I guess you could argue that a AV app isn't supposed to cope with Trojans.
NOD is a bit lighter on system resources, scans quicker but Kaspersky's unpacking engine (for compressed/packed files) is second to none.
Might also want to take a look at Bitdefender 7.2 and Panda Titanium 2004. I'd recommend BD over Panda if you are using Outlook 2003. They don't get along well.
As for free versions, you get what you pay for. In most cases the real-time/on-access scanner is disabled or signature updates aren't released in as timely a manner.
wicked might want to look at version 5.0 released early this month. Most people are having zero issues with multitasking while running a scan. Actually I should correct that... the first scan is very intensive, scans everything, after that it only scans files that have been modified.
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