If you have Windows 7 as Trevor suggested, then I would use MSE and Malwarebytes. MSE will protect most surface items and if anything does get past that, Malwarebytes will go ahead and find it (I recommend always running a Threat Scan). From the systems I have worked on over several years of I.T. work, I have noticed that McAfee, Norton, AVG, and Avast are rather poor choices for security; several times that have not detected anything after a scan that Malwarebytes soon picked up after the initial starting phase. Always attempt to discover a free anti-virus before investing too heavily.
Personally, I would recommend Kapersky (as it was used at University. If it did not work, they would not use it.) or VIPRE (Utilized during High School as the District protection; quite reliable.)
Once that is complete and it shows you a list of PuPs (Potentially Unwanted Programs) go to the button that says "Quarantine All". Then proceed to the "History" tab and "Delete All"; it may require a restart.
As Paracha mentioned, I would use Firefox or Chrome in "Private" or "Incognito Mode" as they: Do not track your browsing, Do not keep a record of your search history, and it clears your cache and cookies (provided you do not have it set to "Always remember/Accept").
If you can download Adblock Plus for Firefox/Chrome, or both if you have it, then do so; it helps quite a bit in removing unwanted Facebook side-of-page ads and those annoying ads before YouTube videos.
Honestly, as long as you never use Internet Explorer and avoid "those sites", your system should be fine. Just watch out for the occasional spam mail, run a scan at least once a week (or more depending on how heavily your browsing is), and never donate money to the Nigerian Prince; there never was a lottery.