White willow bark and aspirin are NOT the same. Dont let anyone tell you otherwise.
Aspirin is fully synthesized now, (hss been for a ehile now) and is actually a lot harsher than the bark. In addition, there's a lot more chemicl compounds in anything herbal, and they all work synergistically with your body to heal you.
Asking what herb you can use for a headache is not really how herbal healing works, and many things must be taken into account. Treatments can vary, depending on what is causng the headache. Is it due to stress? Illness? Infection of some sort causing swelling and pain? I am unable and unwilling to suggest a cure here, as I just don't have enough info. Taking the wrong herb for the wrong ailment can actually make the condition worse, so it pays to be specific and also to not assume that just because it is a natural herbal cure, that it is harmless and safe, because that is not true
And finally, there is ever so many great websites that have a plethora of great herbal information. I would suggest go looking at some, if only to double check any advice some random gives you over the internet....including mine, lol!
Remember, asking questions is fine, but don't take the first answwr you recieve as gospel. Fact check it before acting on it, especially concerning herbs.
Good luck, and I hope such headaches are few and far between for you. :)