Avast is wayyy too heavy on resources. I went ahead and purchased a 5-year license of Faronics Anti-virus. It was rather expensive, (115.00 total) but in the end it was the best choice as I already have a few Faronics Products, (Deep Freeze Enterprise, Anti-Executable, and WinSelect). With faronics products, you get what is called Faronics Core. You can install the various products on as many different PC's as you need, (in my case I have my desktop, 3 laptops, my mom's laptop, and my girlfriend even asked me to install it on hers as she's not exactly tech savvy), and you manage all your "deployments" from Faronics Core, which I installed on my main desktop. As you only need the one license activated through Core, that same license can apply for up to 100 deployments, which is why it was so expensive.
To get a read up on Faronics antivirus, here is the link http://www.faronics.com/products/anti-virus/