There are 2 POD's I can think of to make Islam seriously weaker than in OTL. The first would be the successful resistance of Persia to the Arab onslaught, for then nascent Islam would have had to compete with well-established Zoroastrianism east of the Tigris (it might still have expanded West to the Christian lands of the Mediterranean periphery, though).
Another is an Arab defeat at the battle of Talas in 751 (near present-day Alma-Ata). The Arab victory in OTL led to the spread of Islam throughout Central Asia, in formerly Buddhist lands such as Afghanistan. Remember the Buddhas of Bamiyan, which the Taliban blew up? They hated them because they were a conspicuous reminder that their country had once been a thriving crossroads for Buddhist scholars, pilgrims and artists. (Buddhist sculptors were actually influenced by their Greek colleagues brought along by Alexander the Great when he conquered what was then Bactria, hence the perceptible Hellenistic elements in classical Buddhist statuary).
In OTL, the Arab won against the Chinese armies because at that point in history the Tang dynasty was being undermined from within by conniving frontier officers such as An Lushan and the meddling of the Emperor's favorite concubine Yang Guifei. If you make the dynasty stronger, it likely wins against the Arabs, and the whole of Central Asia remains Buddhist. It also makes it more likely that the Tatars and the Turks don't convert to Islam, and that both Asia Minor and the Caucasus remain Christian (Orthodox, Nestorian or whatever, depending on the area). Obviously, with a non-Muslim Afghanistan, there are no Taliban, and thus probably no al-Qaeda.