Tolerability of a four weeks treatment with Aggrenox® modified release capsules b.i.d, compared to reduced dose during the first two weeks of treatment in a double-blind, randomized controlled parallel group comparison trial among Taiwanese patients with previous TIA's or ischemic stroke.To monitor frequency and intensity of headaches and other safety parameters among Taiwanese patients with previous TIA's or ischemic stroke given Aggrenox using a titration scheme or not