Cleaning / checking a mac for malware

I don't know macs well.  a friend who has a mac wants to make sure his mac is not infected - supposedly his machine was / is being used as a zombie, sending out spam under his hosted email account.

how would you check a mac to see if it's infected.  I know about trend's housecall, eset has a web service to check for malware, but they are for windows only from what I can see.

and what security software would you use to protect the machine going forward?

Thanks