Trust on Anti-virus softwares !!!!!!!!!!

Hi i just want to know that how much u can trust on antivirus softwares ?
coz recently i had discussion with my some friends and a friend asked this i reply him with 90% and he said those trust on anti-virus softwares are fools .He proved this on my PC & more than 1 major softwares with upgradation were installed on my PC.
now what you said for this situation ?
now how can i trust on Anti-virus softwares ?