2 adapters, same subnet, works in XP not in 98

I've got a small network. No DHCP anywhere. NAT on the DSL router. Addresses
are all manually configured in the 192.168.0.xxx range, subnet mask of
255.255.255.0

Just bought a Wireless AP, PCI WiFi card for a desktop, PCMCIA WiFi card for
the laptop.

The laptop, running XP Pro, is quite happy having 2 network connections on
the same subnet (the WiFi and the built in ethernet). Even if they're both
active.

The desktop, running Win 98 SE, won't allow it (it as a PCI ethernet card +
the WiFi adapter). If I disable the TCP-ethernet card then the WiFi
connection works. But if I don't then the WiFi doesn't work. Can't ping the
router or anything. Even without the ethernet cable plugged in. What's (a
bit!) interesting is that while the machine boots, if I get to a command
window and ping the router, then I might get a couple of replies, as long as
the WiFi icon has the cross (i.e. 'not working yet') through it. But as soon
as the WiFi comes alive the connection fails.

I'm struggling to understand this. My searches on the internet give me the
answer that you can't have two adapters on the same subnet.

1. Why not? What is it about TCP/IP addressing that prevents that.

2. I see a slight difference in the network setup windows for XP and 98. It
_seems_ to me as though the two hardware devices in the laptop (ethernet
adapter and WiFi adapter) are bound (?) the same TCP/IP software (?)
adapter. But I'm guessing. Am I on the right lines? What problems, if any,
will this cause.

3. I'll never/hardly ever want to use a wired ethernet connection and a WiFi
connection at the same time, at least not on the same subnet. But at the
moment the Win 98 machine doesn't even allow me to leave the ethernet set
up. So if the WiFi fails, or I just want to move the machine into a
different room to where I _can_ get a cable at it I'll have to reconfigure
it.

The WiFi AP is a D-Link DWL-2000AP+ connected to a NetGear Wired Router/DSL
modem. All the wired machines are also connected to the router.

Any help gratefully received.

Yours, Mike MacSween