In most cases, the file will be located in the following paths;
\Windows\Application Data\Avant Browser
Windows NT/2000/XP/Server 2003
\Documents and Settings\<Your Username>\Application Data\Avant Browser
Note By default, the Application Data folder has it's hidden attribute set to true. If you cannot see this folder, open a random folder and click Tools then Folder Options and click the View tab, in there is an option to "Show all files and folders" (wording may differ depending on your version of Windows).