I have used AVG, Kaspersky, ESET previously but I found all of them to be really intrusive and resource hogs so I have now installed Microsoft security essentials and I am really happy with it since it is light and not intrusive at all.
I at first used Norton 360 and Norton 2012 but I found them to be all a major resource hog which slowed down my computer. In the end I resort to using AVG Antivirus which has been light on the resources and doing quite well.