Jay, try the following:

  • log on as yourself
  • start avast, and hit the button Registration while doing the memory scan
  • reenter the key (copy 'n paste will do great)
  • confirm with OK
  • quit avast
  • restart the machine
  • logon as yourself
  • logoff

Now everything should work OK even if your other family members log on…
BTW do you use the Fast User Switching feature of WinXP?

Thanks for your patience,
Vlk