Get a virus scanner

A virus scanner is an essential piece of software for anyone who uses the Internet, protecting your computer from programs that could destroy your data, steal your private information or even start sending out spam! Many virus scanners also detect other forms of malware, such as trojans and spyware, providing even greater protection for your computer.
Although some virus scanning software costs money (either as a single purchase, or for a subscription to an update service) there are many pieces of free software out there - in many cases, the authors of paid-for virus scanning software will give away a free, limited version for home users (they make most of their money from business users).

H+BEDV provide a free version of their AntiVir anti-virus software. This product can download and install updates for itself, keeping you protected from the lastest threats, but it doesn't do it automatically - you need to tell it to search for updates.

Grisoft also provide a free version of their AVG Antivirus product.

Useful links

AntiVir - free for personal use.
AVG Antivirus - free version available for personal use.

You only need to have 1 virus scanner running - running more than one is unlikely to provide any more protection, and in some cases can cause your computer to slow down and other problems.