Q: how do you check whether you have a virus

As there are none, I don't check for them at all.

All known Trojan Horse malware is blocked by the OS.

If you think you might have installed Adware, you can use these instructions by Apple,

Remove unwanted adware that displays pop-up ads and graphics on your Mac - Apple Support

or use the Malwarebytes Anti-Malware program formerly known as Adware Medic

http://www.adwaremedic.com/index.php

Never use any kind of "anti-virus" or "anti-malware" software on a Mac.

It's best to describe the problem in as much relevant detail as possible, rather than what you think is causing it or how you think it should be solved.

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I have a 2010 iMac and I have Norton on it, just because I have a windows based lap top as well, and purchased Norton to go onto.. I could then add additional computers, so I did.

Anyway.. I just got a message that I have a virus and may be in quarantine.. I'm not sure what this means, and I'm not sure how to check to get rid of it.