Avast is an antivirus software that can be downloaded online. It works in conjunction with your Internet server to protect your computer. No option exists to add trusted sites in Firefox, Chrome or Safari, but it can be a useful practice in Internet Explorer. By adding trusted sites to Internet Explorer, Avast automatically recognises these scripts and allows them to run on your computer without being flagged up.
Open the "Start" menu. Click on "Control Panel," choose "Network and Internet" and select "Internet Options" on the subsequent menus.
Select the "Security" tab. Click on the "Trusted sites" icon and then press the button labelled "Sites."
Type the URL of the website you want to add in the text box provided. Uncheck the box at the bottom labelled "Require server verification." Press "OK." This site is now recognised as safe by Internet Explorer and, consequently, Avast.