i would recommend ClamXav, VirusBarrier, or Avast. i would not recommend MacScan or Norton. these will mostly search for Windows viruses on your Mac as there are currently none for OS X. Windows viruses will not excecute in OS X.
read this whole thread for detailed info on viruses and malware for Mac (well the lack of).
Yep, you might fall into the trap of installing Norton Virus (a resource hog, that slows the system) and Norton Drive Destroyer (which might hurt more than it help due to it being frozen years ago).