This is completely hilarious and ingenious:
My father heard about Vista coming on the news. Since he was interested in getting it, he asked me to obtain it and install it on his computer and he would give some cash in return. I told him I was going to do that, but instead I burned an Ubuntu CD and installed it.
Later, when he came home from work, I showed him his new "Vista" install, complete with the latest Office and Solitaire.
Well, it's been a few days since that and now he says Bill Gates is better than Steve Jobs and brags about how OS X on my iMac is obsolete compared to Vista on his PC.
I will continue with the prank for some weeks, after which I'll tell him the truth and give him back his money.