Saving a copy as in the avast folder/s won't work as the registry entries won't exist in the registry of the fresh XP install. To avoid connecting to the internet unprotected isn't advisable, so you could download the latest version of the avast setup file and either save it to a different partition, 2nd hard disk or copy it to a CD. You could then use that without having to go on-line to get it.
You could also save a copy of the avast4.ini file which contains some of your avast customisations. You should also save the email that contains your registration key, but that isn't so critical as you can reregister.