This article recommends free tools for both Windows and Mac to clean malware off of your computer. For an overview on malware, please see this article.
Malwarebytes is a great tool to scan and remove malware from windows computers. There is a free and paid versions available. In most cases, the free version is sufficient. Visit their website to download and for instructions on how to use this product.
Adwaremedic is a free tool that can be used to clean malware on a Mac computer. Visit their website to download and for instructions on how to use this product.