Another factor is automatic updates which stream down fixes constantly to XP machines- something that 98 and 95 never had. The jumps from 95 to 98 and 98 to XP usually meant a clean, virus-free OS. XP has remained solid because of the ability to update it - something only realistic with larger broadband adoption.
I personally can't wait until your average store-bought computer is some dumb terminal that boots off of a network.
Less questions from friends, acquaintences, etc when all of their sheah's in the cloud.