Try any of the free malware apps linked below. If you have no preference, try the iAntivirus. That's the one I use to scan my Macs about once a month. So far, in 15 years of scanning my Macs, exactly zero infections have been found. Your mileage may vary.
There is no virus. There could be a Trojan if you installed it. Not much chance of anything if you didn't. Apple knows about all the malware that is around and the latest updates prevent them. Unless you turn off your automatic weekly updates, you are all set. The idea of using some downloaded malware scanner is "belt and suspenders" approach.
Most of these pop-up ads for "protect my computer" are legitimate, such as MacKeeper. It is beguiling, persistent, and annoying, but it is no virus. Rarely, though, a Trojan will pop-up and try to sell you something fake. It doesn't have the power to add files to the OS System folder. so worries are minimized. Both Linux and OS X are well protected from viruses.
If you "accidentally" clicked that clean thing, and it was a legitimate "cleaner", it probably removed cookies. That's where the login is kept for online mail, pinterest, youtube etc. It would be really unusual if it damaged the preferences file for the OS X "Mail.app".
@Joe:
Are you able to boil this down for the poor girl? Even I don't know these dozens of tools, except to know that most of them are for Linux.