If you are running Windows, you should have anti-virus software. One of the best is free for personal use. It is:
AVG Anti-virus Free Edition
It includes free anti-virus updates and does a great job of finding and stopping viruses.
The only downside to AVG is that every year or so AVG comes out with an update that requires a fresh install of the new version. And on the website, you need to look for the free version. AVG does push their paid version.
When installing, you don’t need to install their browser plug-in which can needlessly slow things down. Instead, for safety, you should use Firefox when browsing the Internet.
Compared to standard anti-virus software from Norton and McAfee, AVG does both a better job finding viruses and is less resource hungry in my opinion. Often I will find a system that is completely bogged down, not by viruses but by Norton’s rediculously large Internet Security Suite of software.