That may be very tough to do. Most of the simple switching boost designs can only double. Most FETs require at least 10V for a hard turn on. That leaves you using transistors for the lower voltage.
www.siloconchip.com has a nice DC-DC converter project that you could use as an example to modify. The lowest this would go on an imput is about 7V. Most 300W computer power supplies have the the output transformer that could be used as the inductor or a push pull transformer. Use either the 3.3V or 5V center tapped windings and take the output off the 12V winding. Many of these use the TL594 or TL494 simple switcher which is pretty easy to use. Look up the data sheet on it. A few pages down on google (TL494 search) there is a 10W inverter schematic which is close to what you want. Because of low voltage you have to stay away from darlington output and you still need about 6V minimum to run the chip. There are some low voltage switchers, but I don't think these can handle the higher input voltage you want.
