eX Paperclip (they got rid of it at last)
eXclude Pirates (that was the plan, anyway)
eXtra Pillow (useful for when you're waiting for it to boot)
The real answer, however, was revealed by Bill Gates at the Windows XP launch in New York where Regis Philbin staged a mock-up of his famous "Who Wants to be a Millionaire" and asked Mr. Gates what the XP stood for. (originally seen here
REGIS PHILBIN: Bill Gates, what does the XP in Windows XP stand for? Is it extreme power, expediency, experience, supercalifragilistic-XP-alidocious?
BILL GATES: Well, its a pretty easy one. I think Ill go with C.
REGIS PHILBIN: Experience? You say experience, final answer?
BILL GATES: Final answer.
REGIS PHILBIN: All right. And youre right. Bill Gates, youve just won a hundred dollars.