Somethings you can check on:
1. If you are connecting your development computer to the RT controller directly you need to use a ethernet cross cable.
2. If you are connecting the RT to a network/hub/LAN then you need to use a straight ethernet cable
3. Try changing the ethernet cable
4.Try disabling the firewall
5. If you are able to see the IP adress of your RT controller, try to ping it from the development computer
I am having the same symptoms, but it appears that I am missing the network chipset driver. I am in the process of a clean WinXP installation, and thus far, have installed WinXP SP2 from a CD-ROM, and that's it. I downloaded the ftp://ftp.ni.com/support/pxi/pxi-8105/peripherals/PXI-810X%20Drivers.zip file, and as per the instructions, ran the .exe in the sub folders that has an exe file in it. Unfortunately, only the 'chipset' and 'graphics' folders had .exe files. The 'net' folder (sounded promising) had an uninstaller .exe file, and the other folders (inf, NI Trigger router, NI watchdog) contained plnty of files, but no .exe installer programs.