PLC simulator

I've been searching for a useful PLC simulator so I can sketch programs at home when I'm away from school. I've tried going to Siemens and downloading their version off their site but it doesnt seem to be compatible with Vista. Does anyone know of an easy to install basic PLC simulator program. I'm not using a physical PLC, just want something to sketch with.

-Thanks again

Im assuming that you want to be able to simulate and run ladder logic?

either way I think that this might be the best option that I can find

http://www.tri-plc.com/trilogi.htm