Here are a couple of the more successful pranks I have <ahem> “seen” done:
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Set his home page for whatever “the best porn site with the greatest number of pop-ups” is (best to disable the pop-up killers & even better if it is a work computer with corporate filtering that auto reports you to management)
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Get a copy of a really off-beat trade magazine for pig farmers, take out the reader’s service card, fill it out for “<friend’s name> Hog-o-matic Farms”, circle everything, send in. (Note this joke will last for YEARS and comes complete with pig food samples in the mail box).