Note no ant-virus is 100% effective because new viruses are being made everyday and if you do not regularly update your ant-virus in a few months it will be useless to new viruses. These are links to shareware and some freeware anti-virus tools.

Macafee This is the best virus scan I have ever seen.

Norton This one is pretty good I think they have sharware on their page of it.

Virus Research Center I think it gives anti-virus stuff gives some good info and other help to detect one.