I have an iMac running OS X Yosemite. My wife was using it on her account late last night to look for some clip art images. This morning she has told me that on clicking one of the images a box came up and told her there was a virus and she should click the box to download some antivirus software (you know what's coming next!). Unfortunately rather than walking away from this site or asking for help, she clicked "yes" I am now worried that she has downloaded some malware onto my machine. What is the best way of checking and clearing this mess up? She was using her account, can malware still penetrate other user accounts on the same machine? HELP!