The title says it.
I installed a legally purchased copy of XP Pro on my friend's desktop when the HD was destroyed (it had had Vista on it,) only to discover Microsoft tried to charge me $99 to activate it!
So in about a month my friend is going to have a crippled copy of XP Pro running.
Hence the need.
He is a casual user: football scores, online radio, online backgammon, that kinda stuff.