Did you remove Avast with the uninstall utillity before reinstalling?
If not, give it a try.
http://www.avast.com/eng/download/programs/avast_uninstall_util.html

  • Remove Avast with that util.

  • Reboot

  • Install Avast
    If you have XP Pro make sure you are logged in as ADMINISTRATOR
    (not as user with admin rights, but as true administrator)
    If you have XP Home, make sure you are logged in as user of the admin group.

  • Reboot

  • Again login as ADMINISTRATOR

  • Check if Avast is working.

  • Let us know if this is of any help.