Asprin makes platelets less sticky and may prevent the formation of a clot.....it is an anticoagulant and stops the blood clotting.
Heart attacks are invariably due to clots and prophylactic Asprin may reduce the frequency and severity of heart attacks.
Strokes are due 75% to clots BUT 25% to.......bleeds.
Now, aspirin as a preventative has gone out of fashion by most Cardiologists in the U.K and by almost all Cardiologists in the U.S due to the fact that if a stroke is caused by a bleed, then the aspirin will make the bleeding worse......so by and large it is no to prophylactic aspirin.
As for the usefulness of aspirin in a heart attack......I wouldn't bother as most areas in the UK will be well served by non invasive coronary artery stent insertions.