There are already at least two other threads about this in this very forum about Yahoo:
http://forum.avast.com/index.php?topic=74340.0
http://forum.avast.com/index.php?topic=74350.0

And at least another three about ads elsewhere:
http://forum.avast.com/index.php?topic=74347.0
http://forum.avast.com/index.php?topic=74349.0
http://forum.avast.com/index.php?topic=74345.0

Please use search function before starting new thread. http://forum.avast.com/index.php?action=search