Any good antivirus software free for Mac?

[sc_shark]

i have a mackbook pro.

have avast free(just ran out) they want me to register it. i want to delete it. and reinstall the free version. i have no clue how to delete the old version.

i downloaded clean my mac and its a free version but they want me to register and its a trial blah blah blah. so i want ot delete that as well.

any good antivirus software/free for the mac's??