You cant use NFS, unless your grabbing the files of another linux machine. From your description, Im guessing your install files are on one of the Winxp machines. In that case, all you have to do is set up either a HTTP or FTP server on that machine thats pointing to the redhat folder. Then either select the FTP or the HTTP option when the boot disks asks you, and enter the IP address of the machine serving the setup files.
Hope that helps