When it comes to anti-virus programs, you have the good, the bad, and the downright ugly! It would be great if this list remained static, but unfortunately things change. One program may be great today, and absolutely STINK tomorrow. The problem with many security packages today is they're trying to do everything, and in the process, screwing up a lot of things.
Back to the subject at hand. First, there is absolutely no need to purchase anti-virus software. There are some free alternatives out there that are excellent choices and will do a very good job of protecting your system. Just because you paid money for it doesn't mean it's good. Remember, a good anti-virus program is but one tool in your arsenal to keep your computer bug free.
See list of 3rd-party software for a list of recommended programs.
False positives Edit
False positives occur when the anti-virus program sees something in a file that it thinks is a virus or trojan, but it really isn't. With the anti hacking measures built into the Launcher, false positives can and do happen. If you're downloading from Blizzard, and your anti-virus program hits on it, it's most likely a false positive.
Many software companies recommend that you turn off your security software when downloading/installing from trusted sources. This is definitely one of those times. Turn off your security software when you see there's a new patch or you're about to download or install the actual game client from battle.net or your DVD. This will help reduce issues that you may run into during patching and installation - such as corruption of the entire installation which requires a revert and reinstall of the patch.