As a general rule, unless you're downloading stuff while playing, it's worthwhile to close/disable any active (meaning constantly monitoring, as opposed to having to start a scan of your hard drive) antivirus and anti-spyware software. They tend to be resource hogs, and you'll usually have better performance and fewer problems without them.
If you're like me and tend to browse the internet during slow times in game, just use firefox with the noscript extension, and don't download crap that's going to have viruses. Firefox gets essentially zero spyware, and noscript prevents web pages from running the sorts of scripts that, in Internet Explorer and the like, can exploit security loopholes in the software and expose you to viruses and other malicious software. That way, as long as you don't actively download anything nasty, you're perfectly safe. Stay away from porn and warez sites, and you're golden.