Depending on what other software you load at boot time and what applications you use and what the settings are etc, 384 on a 800mhz with XP is very low. I personally have never seen Avast slowing down a system and trust me, I’ve seen many systems. Two things you can do are:

  1. prevent applications loading at boot time that are not really needed for the system to work. http://www.mlin.net/StartupCPL.shtml

  2. ‘Tweak’ you system. Disable all services not needed etc. www.blackviper.com