Microsoft and Trend Micro both make virus detection and removal tools that are available for free online. Microsoft's program is designed as a long-term solution to removing viruses while Trend Micro's software is designed to be run after a virus infects a machine.
Microsoft's Malicious Software Removal Tool and Trend Micro's HouseCall receive regular virus signature updates from their respective companies. The Malicious Software Removal Tool is an installable program that continuously monitors a machine to detect and remove viruses. HouseCall, on the other hand, does not install on a machine. Instead, it runs directly from the downloaded application and removes any viruses that are found in a scan.