script running

[B0LLIKS]

hi all, i know someone will be able to help. since autotrader updated their site, after about 15 mins, sometimes a bit longer of browsing, i get a popup saying something like....a script is running that may make your pc run slow or become unresponsive, do you want to stop it yes/no. which ever button a click on gets rid of the popup for maybe 5 mins then its back. it just happens on autotrader. i have avast runing and on startup there is a little popup that says...script blocker, for a sec or so as the pc is starting up. as long as i stay on autotrader things seem ok, if i want to look on another site i have to close the pc down then start up again as all that happens when i click on a site it just says...connecting, but nothing happens. oh and i know very little about computers.
many thanks....tom