These three tools have both free versions, which work well, and paid versions which work even better.  

Microsoft Updates, if you are using a windows based computer. Vista users can run updates from the program menu and XP users can go to www.Microsoft.com (only using IE as the browser). 

Malwarebytes from Malwarebytes.org. This is the top rated malware removal tool in use today. It solves most minor and medium infections along with many major infections. Fully functional in the "freeware" version but only auto updates and scans in the paid version.

Advanced System Care from IObit is a system utility which includes a registry cleaner, junk file remover. Works well in the free version but works best in the paid version. 

AVAST anit-virus is the preferred free anti-virus and generally preferred over almost all paid AV versions. This changes from time to time as the Malware Industry mounts attacks against different tools. AVG is also used as a good free AV program and the best method is to alternate between these two programs every 4 months.