-No other security software installed.
-Used Avast free for many years. Many years ago, I decided to try McAfee or Norton that was offered by Comcast for free by uninstalling Avast first, it was such bloat ware, I uninstalled it almost immediately, then reinstalled Avast free again. Recently, because of the speed issue, I uninstalled Avast using your uninstall utility, tried MSE, but it pegged my svchost.exe process at 99%, so I uninstalled it. Then tried AVG free, the downstream was at about 36Mbps, but I believe AVG free does not include a Web Shield similar feature, maybe that’s why it was faster, did not like it anyway, so I uninstalled it and reinstalled Avast free again.

This downstream speed issue happens on another hard wired and wireless N systems running XP, with older and current version of Avast free, where the only security software these two systems ever used was Avast free, same thing with them, turn off Web Shield, speed goes back to normal, turn on Web Shield, speed drops tremendously.