Without 9/11 there is no firm move against Al Qaeda and the Bush admin continues to push on the missile shield the way they were before. The economy still goes into the garbage and in 2004 a stronger Democratic field runs against him. My thought is we then see a Hillary Clinton Presidency.
Bush's defeat was almost assured sans 9/11 and the subsequent scare tactics (Vote Bush or WE WILL ALL DIE!!!). His performance was underwhelming until that day and no president who was elected despite losing the popular vote won reelection.
But I think it would be rather difficult for Hillary to run, and much more win, only four years after her husband left office. And let's not forget that many people in the party were still smarting from the 2000 Gore defeat, for which they blamed Bill.