325 mg of aspirin should be given within 48 hours of symptom onset in ischemic stroke; while its good form to give it when appropriate while the patient is still in the ED, per recent guidelines, it is not imperative.
Aspirin should not be given to stroke patients who will receive tPA.
Prior administration of aspirin is not a contraindication to giving tPA, however.
If dysphagia and/or aspiration risk is suspected, give 300 mg of aspirin per rectum.
2007 AHA and ASA Guidelines for the Early Management of Adults with Ischemic Stroke and Guidelines for the Management of Spontaneous Intracerebral Hemorrhage in Adults.