first of all hello everybody!
I’m using avast! 4 home edition,and I have a question about it:
it often happens that, while surfing on the internet, a dialer software is downloaded and started without my permission.is there the possibility to prevent this with avast av?
thanx!! 
ps:if you find my english poor, I apologize…
You must understand that Avast is primary a Anti-Virus software. Nowadays there is something called malware. That is the general name of all applications that are harmfull for a system. This malware is devided in viruses, trojans, worms, spyware, adware, browser hijackers and such.
There is no application, nor will there ever be, that can handle all malware. Therefor you need multiple applications. Click on the link inmysignature and read the page. I think it will give you enough info.