my pc and browser is infected.pls advise

hello,
I am not a techkie guy and i use the free version of avast. for some time now, my mozilla browser use to keep sending alarm of evo gen virus and other thing. when i scan whole pc, no virus is found. i reported it as a false alarm in software and carry on using mozilla.
now its getting worse as pop up are cropping from time to time and i have the impression like my pc has been highjacked. if a format the right way to proceed or should i get the advanced version of avast ?

thks
nitin

Follow these instructions first please: http://forum.avast.com/index.php?topic=53253.0