Did you install WinXP last? If you installed Win98 first, then it wrote the Master Boot Record with only its own boot instructions, excluding all other operating systems. If you installed WinXP second, it should have over-written the MBR with instructions to boot your choice of available Windows operating systems. When you start your machine, the WinXP boot instructions should come up offering you a choice between 98 and XP, with XP as the default first boot option.
Note: If you install Win98 last, it will over-write the WinXP boot instructions and reserve it all for itself. WinXP will be excluded.
Win98 refuses to acknowledge the existence of any other operating system.
WinXP will find & include all other Windows operating systems, but will refuse to acknowledge the existence of any non-Windows operating system. It will include Win98, but exclude Linux, Unix, MacOS, Free BSD, etc.
If you plan to install a Linux OS too, do it last. Linux OS's (& most all other non-Windoes OS's) install with bootloaders (such as LILO or Grub) that will recognize & include Windows in the boot process.
You could also obtain one of a multitude of available bootloader programs to organize the multiple boot process. Partition Magic has one, Norton has one, there are many.