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.
http://www.sophos.com/en-us/products/free-tools/sophos-antivirus-for-mac-home-ed ition.aspx
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.
Jeff