You can ssh from the PC to the mac with a client that supports X forwarding, and then start the apps in that ssh session.
Chances are though, what you want is a login screen to pop up in Xwin32 when you "connect" to the mac, and to do that you'd need to install xdm and configure it as an XDMCP server. Xdm is part of XFree86, but there are replacements like gdm and kdm.