I used AVG free on WinXP for a number of years but I found latterly - this was about 18 months ago - that the virus updates were slowing the computer down, particularly on first booting the computer. I eventually removed it and replaced it with Panda Cloud AV. Panda stores all the virus definitions on line, so there is no updating of the computer so it is almost invisible. It seems to have had some good reviews. I replaced AVG with Panda on my wife's computer (Vista) about two weeks ago as she was having similar problems with AVG slowing the computer down and getting in the way generally.

Very slow booting was a major factor in my decision to try Linux, a decision I have no cause to regret.