First try the PCI NIC in all available slots -- sounds like you did that already. Next, boot without any NIC and see what you have open for IRQs, IO addresses, the ISA NIC probably cannot share an IRQ or IO. Then, put the ISA NIC in and boot to the DOS prompt by pressing F8 when the Starting Windows text appears. In DOS, run the ISA configuration utility for the NIC. (There should be a floppy with the NIC that has a setup.exe on it or look at the card maker website to get it.
Try to put the card in PNP mode if the ISA card has a PNP mode. If the ISA card is a true legacy/Non-PNP card, pic resources that you know are free. (Remember, you just looked, maybe pull the card and look again to make sure, this is critical).
So, exit the ISA utility (almost done). Now reboot and go into your BIOS (If you are in PNP mode, ignore this step). In the BIOS look for a list of interupts that may be set as PNP or Legacy. Set the interrupt that the Non-PNP NIC is on to "Legacy" or "in use" or whatever it happens to be called. Shut it all down, reboot, should work fine.
jerem806