I suggest MSE (Microsoft security essentials -
http://windows.microsoft.com/en-US/windows/security-essentials-download ) for ease of use, and no brainer updating via windows update.
Despite being paid versions, Mcafee and Nortons are both often considered to be worse than any of the free scanners mentioned here.
You also need something to stop malware, which most antivirus products wont do well. Either Malwarebytes antimalware (
http://www.malwarebytes.org/ ) or superantispyware (
http://www.superantispyware.com/ ). I use malwarebyte myself, I find the paid version well worth the cost ($20 USD/lifetime).
Anyone whose computer I work on, I tell them that it will be far cheaper to spend the $20 USD than buy me the beer/dinner it will take for me to fix their computer a second time if the cause is malware.
Paid versions of malware scanners run constantly - so help prevent the malware entering your computer rather than having to do manual scans. Malwarebytes paid version also has an effective block list vs dodgy websites.
I also suggest you don't use Internet explorer. I suggest Firefox (
http://www.mozilla.org/en-US/firefox/new/ ) or Opera (
http://www.opera.com/ ) web browser.
The main (imo) reason to use one of these browsers over Internet Explorer is to install an extension to block ad - which are a common source of crap infecting your computer. AD Block+ is available on firefox and opera and well known for being good.
