Antivirus will allow the detection of known Virus or Trojans. They keyword here is “known”, if it is a new Virus it is likely that your Antivirus product does not know about it, yet. Which means even with Antivirus installed you are still at risk if you down load a file from a non trusted source, like an email sent to you with an attachment, or downloaded from a website.
Just because you have Antivirus installed does not mean you can go crazy downloading files from all over the place, see Step 5 for more recommendations about file downloads.
There are two types of Antivirus, the free versions and the paid for versions. As a minimum you should have a free version installed, but it is often worth paying the extra for the full versions as they typically include extra features that provide extra protection.
The best free Anti-Virus on the planet by a long shot is CYSEC Free Anti-Virus you can down load CYSEC free Antivirus products by clicking here. We have tested this pretty well and it has the best detection rates out of all of the AV products around.
You can find a review of the best paid for Antivirus products here.