Online scanners download a virus/malware definition database and an ActiveX file that allow them to run in the Internet Explorer browser; the download and scan can take a long time, even with a fast Internet connection.
Scanners are practical when you are looking for verification that a problem has been fixed or that a computer is infected. Unfortunately, very few scanners will remove malware. Most importantly, since two antivirus programs should not be installed on a computer at the same time, online scanners are safe to use with your current antivirus program.
Mac users are not immune to malware, it's just that since there are so few of them compared the number of Windows computer, there is much less malware written to attack the iOS. It is still a good idea to have antivirus and malware protection for your iMac or MacBook Pro. You might consider one of the following free programs for OS X(10.6 or later)