Networking Win 7 and XP, got something wrong :-(

Hi. I’ve spent around a week finding information about Networking and have printed out lots of information from this forum and MS. However I still cannot get it right so here a few questions that I hope/am sure you will be able to answer. My setup is as follows, the main PC is running Win 7 Pro 64bit, I also have a PC running Win XP Home and my wife has a laptop also running...