Malwarebytes is a great free program that everybody that works on PCs needs to have in you toolbox. It is available from the following http://www.malwarebytes.org
it does not work the same as normal antivirus program in that you have click and run it, it does not run on every boot. Malwarebytes will clean many infections that pass right by the store bought programs. Give it a try, we use it in shop almost every day.