If you want something that will be reliable and use very little resources, check out
http://www.free-av.com/en/download/1/download_avira_antivir_personal__free_antivirus.htmlAntivir is made in Germany(German Engineering, bro..), it's free version is among the best free antivirus software available..
I have no experience with Avast, however I do hear good things about them too..
I'm just telling that I've been using Antivir for years and it has never once failed me..
It's heuristic engine alone is superior to almost anything you could buy..
The only thing it doesn't do (at least the free version doesn't) is scan files that are emailed to you (before you download them).
However, if the file does have a virus, it'll pick it up and give you options on what to do if the file has malware in it..
Out of all the people I've turned on to Antivir over the years, no one has ever gotten infected..
Personally, I look at a lot of porn...I sweet love porn..I spank it all the time every day all day to porn...Even when my wife's in the room..I don't care...Windows open so the neighbors watch...I have no shame...Guy across the street "Boy, I watched you pull it 10 times today!!"
Ok, I'm totally kidding about all that.. But looking at porn or downloading illegal software is probably the best test for antivirus software short of actually purposely throwing a virus at your system.. People I've done work for a usually fat and single, so you can guess how or why they call me twice a week with problems.. That is.....Until I installed Antivir.. Now, no one calls me except if they have some new hardware to install.. But yeah, Antivir/Avast.. Good stuff.. Try both and see which you like better..