Does anyone know where i can download a really good anti-virus programme from the internet.

I need one that stops viruses from coming on your pc altogether.

i have 'Spybot - search and destroy' but even after destroying errors, i don't notice any change. I also tried installing Ad-aware but when i try to install it it comes up with a weird message.

I just got my Pc, and it is really fast, even with some viruses, but I am noticing it is getting slightly slower, and i wanty to stop viruses before my Pc gets any worse.

If you know anywhere please tell me, or even better send me a link.

Thanks