Similar problem here - except I'm not using Windows RIS but rather Tftp32 and a trial version of nfsAxe.
Although my pxelinux.cfg/... file specifies both "nodhcp" and "ip= .. gateway= .. netmask= .. dns=.." and although I am using the vmlinuz & miniroot.gz files from Knoppix's own terminal server, it still appears to be attempting to DHCP. Seriously, WTF? (Instructions for PXE-booting gleamed from Here.)
These DHCP attempts are very very rapid - perhaps instant. Tftpd32 doesn't log any DHCP attempts, despite initially succeeding in issuing an address to the PXE client in the NIC and dumping vmlinuz & miniroot.gz into its RAM.
In the startup I get:
Loading network device module(s)
Found network device(s) handled by 3c59x.ko.
Found network device(s) handled by 8139too.ko.
Found network device(s) handled by eepro100.ko.
Found network device(s) handled by sis900.ko.
Found network device(s) handled by tulip.ko.
All these appear as you would expect in the "Modules loaded:" at the end of the DHCP attempts.
My client machine - which successfully boots over PXE with the CD as its main data source with full network access - is running a Via EPIA5000 motherboard with an onboard VIA NIC with integrated PXE loader.
Any help gratefully appreciated - please bear in mind I have a 'doze PC and a PC with no HDD, let alone an installation of Linux =)