I will not tell you the "best" AV, because it's very subjective. ALL antivirus programs have waylaid PC installs; and all of them have probably worked perfect on other PC's as well. There simply isn't one that is better than the others. You can find online reviews testing and finding every one good and bad, the reviews are nearly worthless.
....Some people of course wait until their PC's are already infected to install AV programs, and that often leads to a world of sheet--and a lot of AV will make a mess of everything trying to install on a infected OS,,, so I don't include that in my comment here. What I mean is, I have seen or heard pretty much EVERY AV program out there jack up a clean system. So cross your fingers, make a save point and dive right in! ;)
That said I will say a few things I do know:
----I used to have F-Secure on my desktop up until a couple months ago and didn't like it because my preferred firewall program is ZoneAlarm, and F-Secure isn't compatible with that. There is a trick you can do to install them both but one or the other may not work properly. This information is accurate as of a couple months ago. .....I got F-Secure antivirus (not the whole security thing, which I usually don't bother with getting the whole packages, just the AV scanners) and F-Secure included a "spyware protection" thing that is basically the same as AdAware, just with F-Secure's name on it. F-Secure AV also included a "registry protection utility" that would inform you on boot if any registry entries had been [suspiciously] altered, and would ask if you wanted to undo them.
----I have a newer laptop that had a free install of McAfee trial on it(which seemed to work well enough) but when the 30-day trial was out, it was not renewable as a regular McAfee install. Pardon my French but Fuck That. I refused to install another full-pay year of McAfee just out of spite (-also after I uninstalled McAfee and rebooted, there were still at least 3 McAfee processes in there on startup, that I need to go in and hunt down sooner or later; I just bolcked them in ZoneAlarm for now).
----What I got (for both these PC's) was a 3-license CD for Norton AV 2006, for about $90. It installed on both just fine, but it has one main problem (for me) right now: inbound scanning of server-based email doesn't work on either PC--the AV program prevents your username and password from being sent to the mail server. You can still scan email attachments, it just doesn't do it automatically like it should. You must do it manually--but if you aren't using Outlook anyway, activeX stuff won't run on preview anyway, so it is still workable, you just have to be aware of that bit of info. This (inbound email scanning) problem is on both 2K and XP systems running all versions of Norton AV 2006 (the single program and the bundled "security suite" packages) and doesn't seem to depend on the email client you are using (it's a problem for all of them). At the place where I got it, Norton and McAfee were both running right at the same prices; $49 for single-user or $90 for 3-license.
----Also I have noticed that I have to exclude my OSx86 image folders from full-scans, or the Norton AV 2006 engine conks out and shuts down. It's odd but that's not a big deal really.
----Also what I dislike about most of the "big" programs like McAfee and Norton is that they are default set to be very "in your face" about anything they find is a security risk, like not having Windows Updates and Windows Firewall turned on. It can take a bit of searching to find out how to turn that crap off. Norton for example has something called "Norton Protection Center" that runs in the systray or task bar and doesn't really tell you much of anything--it's for managing the "internet security suite", but I only got the AV. So it doesn't tell me anything that the AV systray icon wouldn't anyway,,,, but I can't figure out how to get rid of the Protection Center icon. >:(
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