everytime i open avast, or turn on my computer this message comes up 4 times in a row “Either there is no default mail client or the mail client can not fulfill the messaging request”… i do not use a pop server email… i use yahoo, and i do not know how to turn the mail option off. is there anythin gi can do to make it stop giving me this message. It is rather annoying. 
You can disable the Internet Mail protection provider - Click on the Avast icon in your system tray, when the On-access Scanner window is displayed, select the Internet Mail provider and click Terminate. This will disable the provider, if you have other providers that you don’t require, such as Outlook you can terminate them.
This should hopefully get rid of your nagging messages, you may have to run the Mail Protection Wizard to remove email protection.
HTH David