1st, get ClamXav
. It's donation ware, and is a good enough anti-virus that Apple used it for their server version of MacOS X. Check your system with it, especially your user files.
2nd, get LittleSnitch
. It checks for outgoing calls, and allows you to validate (or not) anything it does not recognize. If you have a Trojan Horse sending data to something you don't care to allow, LittleSnitch will alert you.
3rd, if you can, upgrade to Lion. Apple has quietly introduced a hidden utility that tracks and destroys certain Trojan Horses, and attempts to keep it updated, behind the scenes, to keep up with the malware developers.