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And Here for keeping your Mac happy...
You will find this User Tip on Viruses, Trojan Detection and Removal, as well as general Internet Security and Privacy, useful:
The User Tip (which you are welcome to print out and retain for future reference) seeks to offer guidance on the main security threats and how to avoid them.
That is a different question all together. Many problems are caused by not keeping sufficient free space on your start-up drive, which should be at least 25GB or more.
What's the best free virus program for the Mac?Not just for viruses, but removing all the duplicate folders and tuning up my mac.
I am not aware of any program that meets those needs and is free. If there was it would probably cause you as many problems as the ones they sell.
Thomas Reed lists some free alternatives in his article http://www.reedcorner.net/beware-mackeeper/ .
Randy Singer has some ideas in this article http://www.macattorney.com/ts.html#software .
A sluggish and slow Mac can have nothing to do with malware and/or duplicate files.
If you realy want a free anti-malware tool clamav (already mentinoed) and the free Sophos product are reputable solutions.
Common casues of slowdowns are not enough memory, and not enough free HD space on your boot drive.
There will be varying opinions, but sometimes ensureing you have a reliable backup (better yet, 2) of your data, erasing the HD and re-installing the OS can be a good idea. It is certainly overkill in some/many situations, but you can spend a lot of time trying to figure out exactly why you Mac is slowing down if there's some sort of software issue, and not just a lack of memory, storage, CPU speed, etc...
You may also consider posting as much info as possible about your setup and when it slows down. There could certainly be a simple fix, but you'll need to privide the good folks here more info....ther are a lot of folks who know way more than I do who may give you a more simple and elegant solution.