if you look at the cochrane library, maybe this review will be of use in discussing the background information: Clopidogrel plus aspirin versus aspirin alone for preventing cardiovascular disease
Please look at the latest ESC guidelines on CV prevention in 2012. Aspirin decreases the incidence of myocardial infarction and (less clear) cerebra vascular accidents in all cases, lower and higher risk patients. The decrease is constant and is around 15%.
But ....
If your baseline risk is low or intermediate you cause more harm than good. In 1ary prevention you have I major bleeding for 2 saved ischaemic events. The cause/benefit is problematic. For this reason we do not prescribe aspirin anymore in 1ary prevention (with maybe the exception of very high risk patients) but keep it as the first line in secondary prevention. What we do is to decrease CV risk with statins. This is a much safer and established approach.