There might be multiple things contributing to why your phone is running slow. What kind of phone do you have and what Android version? What angelo said were some good ones. Keep in mind some androids (like my Jellybean) can get flooded with extraneous files which can really gunk up your device. Also, having too many things running in the background can bog it down as well. It may not be a virus.
I use Zoner Antivirus. Avast works good as well. I also use Slw cache widget.
If you start using applications like the ones angelo and I suggested and your device is still running slow, you may want to do some troubleshooting. My Data Manager has done wonders for me as well as Mobiwol no root firewall. With them you can find out what is consuming data when and tells you if you are the one that is using it. The firewall can tell to who is calling out to what and when and gives you options on how to control the data flow on each application. Between the 2, you can make a very tight and efficient machine.
All of these are free. Hope this helps.
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