Could be anything
Seems to be a windows problem but anyway here's what I'd do..
There's no firewall on the machine no?
First thing I'd try is in the network cards settings uninstall tcp/ip protocol in case something has buggered up the reinstall it again.
And after that Here's what I'd do even though it's a bit of a pain...
Win 98 has add/remove hardware apple ya?
Using that applet (or in device manager) I'd remove each network device including the modem, power down then physically remove everything from the machine, boot up again, power down again and add the brand new card, power up and install the driver's if need be
If it still don't work look in the device manager thing and see if the cards got an exclamation mark next to it if so see what it's saying the problem is and look in System Information see if can gleam any info from there about conflicts or whatever
If not I'd try the tcp/ip thing again and after all that if it ain't working take it back and ask the blokes in the shop to show you it working - or do that in the first place leaving everything as is the way you got it back.