Macs have never been immune to viruses or malware. No OS is immune. You have much to learn about malware as it relates to Macs. Also, Sophos is not recommended, as it can actually increase your Mac's vulnerability. The malware you installed yourself is a variant of the MacDefender trojan. The only way you had that installed on your Mac is if you were foolish enough to install it yourself, or gave someone else access to your Mac. No antivirus can protect a Mac from its greatest threat: the user.
How about stopping said known trojan hosting site from yourself by using OpenDNS (they have a instructional video) because they use the spam reporting site PhishTank reporting site to block known Trojan hosting sites.
So this way you never visit known trojan hosting sites. Plus never download a program from a "forum" you never been to before and also if you have never seen this poster before. Also don't trust IRC program sharing sites because there have been Trojans embedded in cracked Mac programs before.