Yes, the free is fine. I suggest you do not tick the option to 'enable skins' in the setup panel, but leave it on the simple interface. There is the option to enable a bootup scan. Reboot when you have selected it, and it will run. It will not run again until you select the option.
WARNING: If you have a deep-rooted infection, it is more than possible that the AVAST boot scan will delete some of your Windows system files IF they are infected. You correct this with your Windows XP disc - you need to run sfc from the 'run' box.