Avast drops broadband speed test by 50 Mbps

I have a 100 Mbps connection but was never getting anything above 50 … until I disabled Avast. Then it went to 90+. I tried AVG which was 10 Mbps better than Avast.

How can I be sure I’m getting the internet speed I’m paying for?
http://ask-leo.com/how_can_i_be_sure_im_getting_the_internet_speed_im_paying_for.html

also the way antivirus and the speed test work…it may not show correct result

best way is to download a big file…like xp service pack3 and take the time
then disable avast, download again and take the time…

how big is the difference?