well appart from the fact that all of the windows versions have been meen lol...
id say check all your configurations; because xp is obviously newer, it can take more information; for instance, if your compters name is longer than the 8 characters that win98 can handle, it will jus say "computer says no" (little britain, BBC3)
the other problem it could be is IP broadcasting (DHCP). if your wireless router isnt sending out IP addresses, then the computers wont like that either
Basicaly, wireless networking is always a bit fiddely, so u just need to work at it till the stupid thing works lol
hope that helps a bit
SL