Hi Dramaqueen
There are various free Antivirus solutions out there but the one I use and is completely free is Avast 4 Its a top quality scanner and gives you on access scanning meaning it will stop viruses and Trojans from even installing and preventing your machine from being taken over by trojans or hackers..
Its not resource hungry and you won't even know its there, it automatically updates virus definitions twice a day and tells you when it does..
You can get it here..
http://www.avast.com/eng/download-avast-home.html
Just follow these instructions to register the product when you have installed it, and you will have free updates for 12 months..
avast! antivirus Home Edition is FREE to use but it
is necessary to register before the end of the initial 60 day trial period. Following the registration you will receive by e-mail a license key valid for a period of 1 year. After you have downloaded and installed the program, the license key must be inserted into it within 60 days. The registration process is very easy, and it will take you only a couple of minutes.
Hope that helps you out..