Dear here,
I just installed Avast Home V4.8, but one fed up thing occured, when i started on-access scanner, i couldn’t log on any website, so i stopped on-access scanner, i can freely log on website, which confused me, please tell me how to solve it? Thanks in advance for your help.
My operating system is Vista, IE version is IE7
Welcome to the forums, MarkYuan. 
This is most likely a firewall problem. Avast must be allowed through the firewall.
What is your firewall?
Does your firewall allow ashWebSv.exe internet access ?
- If it does delete the entry for it and reconnect to the internet, this will force the firewall to ask permission again.
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
Thanks, my firewall is windows self, but in the process of installing, the system didn’t let me to give authroty to pass the firewall.
The windows firewall in Vista outbound protection is disabled by default so shouldn’t interfere, which makes my second question about a previous AV the likely cause.
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
No any version in my system, i firstly installed the Avast.
If you are sure there has never been another AV installed (some new systems come pre-installed with a trial, etc.) then that should eliminate another possibility.
The only other thing I can think of is are you using a proxy to browser the internet ?
Have you tried a different browser like firefox or opera ?
As a temporary measure rather than disable all on-access scanning, you could Terminate only the Web Shield and that would have the desired effect.