1st of all, having two anti-viruses always leads to trouble, you must uninstall one (keep clamxav)
2nd of all, at this point in time there are no viruses for Mac OS X, a few trojans but no viruses and there is really no point for an antivirus
GGJstudios is right. Common sense is the best weapon, for example if when you download a movie it asks for your password obviously you know something is up and to delete it or if the size is significantly smaller/bigger than what it should be.