It isn't "an XP problem" as such. I have my MacMini, XP Desktop and XP laptop all currently connected wirelessly, but normally only have the XP laptop connecting this way, which isn't a problem. No individual computer in the network should have any effect on the ability of another computer to establish a wireless connection with the router.
I don't recall what was discussed previously, but if you're using the wireless adapter's software, try using Windows' wireless software instead. If that doesn't help, possibly a "hard" reset of the router back to factory default settings, which involves setting it all up again :-(