I second the vote for Avast Home. The registration is kind of quirky, but overall, it is the best I have used...even over a good number of paid services.
I used antiviruses like avast, avira, panda and so on. Now a days I am using internet security essentials(ISE). This is free and I have found quite handy. It is quite good and light. Better than avast in my opinion.
Avast is the best one.. Go for Avast Free Home Edition... If you plan to pay, then i recommend Kaspersky..
I use Microsoft Security Essentials on my Windows PC. So far so good and it's been over a year. Free to use and I haven't run into any problems.