Installation problem of antivirus

Hello,
previously there was avira anti virus in my laptop ..but suddenly it was not opening to scan the full system . its icon was not showing ..
then i uninstalled it and again i insatll avg antivirus but it was also not opening ... just its icon comes one time on desktop and goes on....
after that i typed on google the word "antivirus "the page of mozila firefox also goes on ...
what is the problem i m not getting
plz help .....