This is not a hard thing to do. There are a lot of full featured antivirus out there that work really good and will not cost you a penny.
First of all you need to know the diffrence between an Antivirus software and an Antispyware software they are two different things and sometimes they come bundeled together but other times you will need to install 2 difrerent programs.
Here is a list of some of the most used Free Antivirus you can find and download from the net.
