I have a Dell laptop which pre-installed McAfee, because I don't want to pay for the license, so it becomes quite a nuisance when message windows pop-up several times a day, asking me to register, although it's still within the free trial period.
Spyware Doctor with Anti-Virus that comes with Googlepack is good, because it has basic function to clean up when find infections. The one you download from official website doesn't provide this service.
As far as my own experience goes, the best,antivirus software is Microsoft Security Essentials, although I don't know if it's most popular. Microsoft Security Essentials runs quietly and efficiently in the background so that you are free to use your Windows-based PC the way you want—without interruptions or long computer wait times. This is just what I want, because to be honest I really don't care much if my pc is PROTECTED or not, as long as the Security Centre tells me YOUR PC IS NOW PROTECTED, gives me peace of mind and leaves me alone to do my job without the pop-up windows interfering my thought.
Of course, your Windows system has to be genuine. Many of my Chinese fellows just buy a naked machine, and pay 10 -20 RMB (about $2-4) for a CD or DVD which includes 'genuine windows system' and many hacking tools, so when I recommend them to try Microsoft Security Essentials, they just shake head and smile.