I regularly veiw my emails using Firefox.But avast hasen’t scanned a single mail because in the mail shield it says ‘scan count 0’.Could please help me with this problem Avast team. 8)
I think that that was normal. 
Definitely is normal yes, because you are using your BROWSER to view your e-mails. Meaning it is the WEB SHIELD that does the scanning …
Then what does mail shield scan and if there are viruses in the email will web shield clean them.
The Internet Mail provider scans emails received/sent by email clients that use the standard POP/SMTP servers to access email accounts and download the mail to be held locally on your system. Such mail clients are Outlook Express, Eudora, Thunderbird etc. In this case it understands the format of the emails; it decodes and scans them. It can prevent the complete receipt of compromised emails or just remove an infected email body part such as an attachment before the message is committed to the message store on the local system or before an infected message is sent out.
The Internet Mail provider is not involved at all in the reading or sending of emails via a Web browser. This does not involve the transfer of the email to a message store on your local system. The viewing of messages via Web browser is no different from the viewing of any other Web page via the browser and all are scanned by the Webshield.
The downloading of email attachments via the Web browser should be treated in exactly the same way as any other downloads from the Web - avast has no way of knowing that this download is from your email store - it is just a Web download. If your browser provides a capability for downloads to be scanned (like the Download Statusbar extension for Firefox) automatically by avast then that helps. Otherwise you will be relying on the standard shield to scan the downloaded file when you access it.
Thanks for excellent info. 8)
I have one more problem.I have the network shield and it is set to normal and it also says ‘last count 0’, anyways to help me resolve this problem.Also changing the network shield to ‘high’ will this slow down my internet connection because this type to shield will provide good protection against internet worms.
Changing the sensitivity won’t do anything, it is more to look the same as other providers.
The 0 will only ever change if the network shield detects an intrusion attempt and then you would know as avast would alarm as normal.
In the Standard shield of Avast in the advanced tab , it says ‘do not scan system libraries’ and it checked.What happens if I uncheck this box, will it put a risk to my system or increase the security.
Obviously from not scanning something to scanning something will increase security rather than put it at risk. But, scanning system dlls could also impact performance as many system dlls are opened and I doubt the option would be there if it were a great risk.
That is a default option no to scan them and is checked on my system.