Which Antivirus? Are they really necessary?

I'm always curious about what those antiviruses really do. Everytime I set up a new antivirus program on my computer, i encounter some problems. My computer gets slower, they find a virus and i can't fix it!! And a bit later, my computer breaks down. This makes me really an anti-antivirus. Then i decide not to use any of them. I just check my computers from time to time with online virus scanners or free antivirus programs. And i feel safe. By the way my computer works faster and with no errors...:)

i hope nobody sends me a virus knowing that i dont have an antivirus program :(.

here is the list of antiviruses which i used and found useful.

-Kaspersky online virus Scanner
-Avira Antivir Personal Free Antivirus