You can use your Win98 PC to download all SP1 + Critical Updates for WinXP from the Windows Update Catalog site:
http://v4.windowsupdate.microsoft.com/catalog/en/default.asp

These updates can be saved to your Win98 HDD, then you can either burn a copy and install it on your XP PC or if both PCs are networked browse to the Win98 PC and install from there.