I just ran in to my first OS X system with a malware problem. The machine had a memory-resident program called Genieo installed which hijacked both Home Page and Search settings for all installed browsers and seems to have been displaying ads in spite of adblock+ for Firefox, Chrome and Safari.
The user was flummoxed as to how to remove it, though doing so really wasn't any more work than the state of Windows Spyware Removal in, oh, 1999 or so (stop the application, uninstall it, change browser settings back).
The computer in question had both an outdated version of Java and Frostwire installed, either of which could have been the initial infection source. Frostwire appears to be the primary function of the computer as far as I can tell. I'm sure if she'd had a Windows PC, it would've been the Whore of Babylon as far as Internet Herpes go.