I downloaded mame (last version), and I noticed very accurated and good looking resized modes, in fact you can resize your game window in real time and the image obtained is really good.
It uses 'stretch using hardware' for this purpose, what is this?, how it works?, sorry, but I have no idea.
I think is 3d accelerated hardware related, it's possible to implement resized windowed modes using stretch by hardware in allegro, or allegroGL?

It probably draws the "screen" to a texture and then renders the texture on a polygon under OpenGL or DirectX, with nice filters and whatnot.

Then, I will need allegroGL to implement stretch by hardware
My game is being developed in allegro.

Allegro currently doesn't handle resizable windows (look for it sometime in 4.3, people are working on this).
Stretching of imagies is currently done in software, although there is no principle reason why it couldn't use hardware acceleration. I think AllegroGL may do that now, but I'm not sure. You'll have to check out the CVS version of AGL to make sure.

You don't need 3D acceleration for that. It can be done using overlay surfaces.

Care to update the overlay driver?