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. :)
Aspirin is also a lot more effective than white willow bark and whether or not something is natural or synthesized shouldn't be a factor in decision making unless one is allergic to chemicals in synthetic medicine (all of which have warning labels and dosage guides; aspirin is a lot more potent than white willow bark, so the same size will not have the same dosage, even).
The basic idea for herbalism is to use what you can have in the kitchen until you have proper knowledge of medicinal herbalism. While the warning above is good in that you should double check information and make sure they are from valid sources, not all herbs are going to have quite as serious of an effect as described (especially herbs used in teas and those used for seasonings). While useful, avoid webmd.com if you look too much into side effects. Without proper medical knowledge, in can scare some folk into believing that their ailment is much more serious than it often truly is.
While you are correct that White Willow Bark and Advil are made of different chemicals, I think Lark was meaning that the effects on the body are similar. Advil is pretty much a man made version of salicin, which is the chemical in White Willow Bark that works to alleviate inflammation. Now, keep in mind that both Advil and White Willow Bark should be avoided if you are pregnant or nursing, as both salicin and ibuprofen are harmful to developing children.
On a side note, if you are able to use either Advil or White Willow Bark, keep in mind that Advil works faster but White Willow Bark, while being slower to effect the body, has longer lasting effects.