I've tried the PEQui 8051 Simulator that Tedd has told me and I'd like to know opinions about people who have already used it. I think it's new so if anyone wants to try it, its download is available at http://www.pequilink.cjb.net
the most interesting is that it's a very nice program and completely free. People don't usually develop complex windows programs and distribute them for free like that...