If you're just using limewire for downloading music, then use mp3raid.com. As far as movies go, those are better off paid for. On a Mac you shouldn't have any problems with either.
Probably not. They may be using slightly better antivirus if programs like those even use something like that. But it's all shared from another person with good or bad intentions. I wouldn't risk an expensive computer trying to test it. Goodluck.