Follow Macman45's instructions. It can't be a virus, since none exist
Macs are not immune to malware, but no true viruses exist in the wild that can run on Mac OS X, and there never have been any since it was released 10 years ago. You cannot infect your Mac simply by visiting a website, unzipping a file, opening an email attachment or joining a network. The only malware in the wild that can affect Mac OS X is a handful of trojans, which cannot infect your Mac unless you actively install them, and they can be easily avoided with some basic education, common sense and care in what software you install. Also, Mac OS X Snow Leopard and Lion have anti-malware protection built in, further reducing the need for 3rd party antivirus apps.