It’s only available in Windows 2k/XP.
Right click the ‘a’ blue icon on the system tray.
Start avast antivirus (2nd option)
Right click the skin (on a clean part of it, not over a button for instance)
Choose ‘Schedule boot-time scanning’
Choose if you want (or not) to scan archive files (*.zip, *.arg for instance)
Choose the default desired option to take if a virus were found
Boot
Hope this helps. 