Um, I may have fixed your problem. I found some code in an old RPG I half made.

onClipEvent (load) {

	speed = 4;

	radius = 8;

}

onClipEvent (enterFrame) {

	if (Key.isDown(Key.LEFT)) {

		if (!_root.wall.hitTest(_x-speed-radius, _y, true)) {

			_x -= speed;

		}

	}

	if (Key.isDown(Key.RIGHT)) {

		if (!_root.wall.hitTest(_x+speed+radius, _y, true)) {

			_x += speed;

		}

	}

	if (Key.isDown(Key.UP)) {

		if (!_root.wall.hitTest(_x, _y-speed-radius, true)) {

			_y -= speed;

		}

	}

	if (Key.isDown(Key.DOWN)) {

		if (!_root.wall.hitTest(_x, _y+speed+radius, true)) {

			_y += speed;

		}

	}

}

It goes in the player and ALL the walls are given the instance 'wall'.

Wait, what kind of game is it? Shooting, Adventure, Sports? Tell me and i might be able to help.