I've recently got a NetGear USB Wifi adapter working on a Win98SE machine. It was supplied with NT/2000/XP drivers only, but Win98 drivers and software were available from the NetGear website.
I did have quite a few issues with it to start with, but I think that was my fault, having tried to install the XP drivers first... I ended up re-installing 98. It works like a dream now though.
PM me if you'd like more info.
Also, note that you can get a PCI wifi card quite cheaply, assuming that the machine has a spare slot. You may well have the same issues with drivers etc, but USB isn't the only way to go...
cbl.