It's been a while here to but I think this sounds like what I used to run into, especially since you modified the hardware configuration.
Some older hardware required specific IRQ and Memory address space. If you have 2 conflicting ones (trying to use the same IRQ for example) you need to change one. Look in the device manager for But some devices will only work correctly with certain values.

Try changing the value (IRQ or DMA setting) of the added hardware (likely a modem or card for CD)
Or try removing them and see if it clears up the resource conflict. Then you will know the network card works, and add one device back until the conflict re appears and you will have you answer as to what device is causing it.

Computer System Resource Information

It could also try turnig off IRQ steering.
Windows 95/98 Support-irq steering