How to simulate a circuit with LM2576 in it?
TI's web site also offers the Webench designer (brought over from the National web site when TI and National merged a while back). You can find it right on their home page and pick the "Power" tab.

The Webench designer helps you design a switching VR, shows you a schematic, BOM with suggested standard components and provides budgetary cost comparisons between competing designs. The tool also will simulate the design and show analysis results.