Not unless the desktop you want to isolate is on a different subnet at a minimum. Even that is no guarantee, but it's at least a start. The best setup would be to not even connect the desktop you want isolated to the same router, but to get it it's own dedicated network connection. Because if this system is connected to the same router as everyone else, you are effectively all on the same network, and that means it's possible for someone to find that system and decide to start poking around. Your average home router simply is not equipped to provide anything even approaching the kind of security you want, and I suspect you'd have a severe case of sticker shock induced whiplash if you went looking for one that did.
I would like to add wireless access for public use. I want ensure that a desktop private computer (WinXP SP3)is isolated from an unencrypted wireless public access point. Is it possible to do this by connecting the desktop and LAN output of wireless router in an unmanaged switch and the router to DSL modem?