avast is a resident on-access scanner, it is always on and scans files prior to them being opened. It also scans some of the files during boot. You can initiate scans manually 'on-demand'
You can also schedule a boot-time scan (if this is what you mean ?) to happen before windows starts on your next boot, however this is only available with winNT, win2k, winXP, but not for Vista.
If you have XP or Win2k, you could enable a boot time scan. Right click the avast icon, select Start avast! Antivirus, Menu, 'Schedule boot-time scan...' This option will be greyed out if your OS and avast doesn't support it.