If you're having trouble installing network cards in your Windows 9x computers because Plug and Play (PnP) isn't playing, install them in non-Plug and Play mode. Knowledge of the current network card setup will save you a lot of time.
Assuming the network card is not an old jumper-set card, nothing is visible that tells you the card's current settings. While you could make a last-ditch guess that it's still set to the factory defaults, this is useful only if you have the manual or can find a relevant Web page to tell you what the defaults are.
Most network cards have a setup program, but these may not be nicely drawn Windows efforts. For example, the 3COM ISA 10-Mb 3c5x9 family has a DOS configuration program called 3c5x9cfg.exe. It's not pretty, but it sure does its job.
The program should allow you to set and note the IRQ and the I/O port. Typical settings are 5 and 220, respectively. Hopefully, the program will indicate a setting that's already in use with an asterisk or another mark.
Once the settings are noted and you're sure there isn't a conflict according to the setup program, reboot Windows and install the network card as an Adapter in Control Panel's Network applet. When the driver installs, try to select the non-Plug and Play driver, which should be clearly labeled or identified as "legacy."
You'll receive a prompt to enter the IRQ and I/O details that you noted earlier. If there are any conflicts, Windows should report these. However, keep in mind that Windows has been known to inaccurately report conflicts at this stage.
Restart Windows and see if the card has installed properly. Go to the Device Manager in the System applet of Control Panel, select the network card, open its Properties, and click the Resources tab. The golden words you're looking for are No Conflicts in the Conflicting Device list.
If conflicts are listed, use the Properties pages of the Computer object in Device Manager to find out what's in conflict. Then use the network card's setup program once again to move the conflicting resource setting to a free one.