I also like ClamAV, it's free, and has the advantage that front ends are available for all major platforms Windows, Linux, OSX, BSD - you name it.
For a nice simple MS Windows GUI port using the ClamAV libs try :
(or the portable MS Windows version at portableapps.com)
The detection features in ClamAV have been upgraded in the more recent versions, but it is still basically a signature based scanner.
On Windows at least, the ClamWin program can be set to either only detect malware, or to also try to wipe the virus or move it to a special quarantine directory. In *nix environments this simple feature could easily be scripted with ClamAV, but such measures are not very effective in dealing with modern sophisticated malware.
A lot of the nasty malware out their in windows will use system locks to keep anti-virus programs from being able to do anything with the file.
I think that this is why some folks down check ClamWin and ClamAV, not so much because they won't detect something, but because they are not much help in removing them.
In Windoz, there are some really nasty bugs out there that, even after being deleted manually, just recreate themselves from another hidden randomly named instance somewhere else on your system.
So, the commercial anti-virus applications, focus much more on 'cleaning' because they know that most folks are really dumb about doing backups, and are desperate for a 'magic bullet' that will rescue them from the consequences of their carelessness.
In Linux, there is no reason that such really bad bugs could not also be created, nasty mutagenic polymorphic orginisms that are difficult to detect and damn near impossible to remove, but fortunatly the *nix community has not been the main target of commercial quality malware so far, because these attacks target mainly the desktop market (where Linux is only a tiny percentage).
The lack of sophisticated 'cleaning' in ClamWin and ClamAV doesn't bother me much, because I always have full hard drive backups available. So, when a virus proves difficult to remove (as is frequently the case in Windows XP), I just wipe the full hard drive partition and restore the system from the last clean backup (including the Master Boot Record).
I am careful about what applications I run, and have had pretty good luck avoiding malware myself, but one of my friends XP boxes gets infected about three times a month. I have finally shown him how to do the 'wipe and restore' trick, and now he never has a problem restoring his system.
Why fiddle around with 15 different anti-virus apps, trying to find that 'magic bullet' that will finally remove a really nasty piece of spyware, when wiping and restoring the hard drive only takes a few minutes?
The thing I like MOST about both ClamWin and ClamAV, is that they are quick to set up and have a very very low system footprint.
For example on Windows, where the threat is greater, if you would feel safer also running another anti-virus, at least ClamWin won't fight with it.
In fact, in windows, you don't even have to install ClamWin to have it do a scan on the system. You can get a nice 100% portable version that will run from a USB Flash drive (portable ClamWin from portableapps.com)
Linux has a much better record as far as malware goes (stronger system, fewer attacks), and with Vista, Microsoft is trying to play catchup, and says they have improved security.
If so, something simple like ClamAV on the *nix side, and ClamWin on the Windows side, may be all that you need.