If you want to set up a dual boot with Windows XP and 98, then the older OS should always be installed
first. If you install Windows 98 (in a seperate HDD partition) after installing XP, your boot record will be
overwritten, and your installation of XP will become inaccessible. It would still be there, with all your
programs, documents and data, but it would require a repair install to make it bootable again.
If you want to go ahead with setting a dual boot system,
carefully follow these steps . . .

Back up ALL your important documents and data to CD-R or another HDD.

Start up your computer with your Windows 98SE floppy boot diskette.

Use FDISK to remove all of the exisiting partitions on your HDD.

Reboot again with the Windows 98 boot floppy when prompted.

Select the "Start computer with CD-ROM support". option.

Use FDISK to create one Primary DOS partition. Reboot.

Start your PC again with your Windows 98 boot floppy.

Format the partition with the "FORMAT C:" command.

Install Windows 98 Second Edition from a legit CD.

Use Partiton Magic to create an NTFS partition.

Reboot with your original Windows XP CD.

Install Windows XP on the NTFS partition.

Reinstall all your software applications.

Restore all your backed up data.
Hope that helps.
UserAgent: Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.8b2) Gecko/20050504
SeaMonkey = Swiss Army Knife: It's versatile, reliable, and contains useful tools.
Windows Internet Explorer = Old Swiss Cheese: Full of holes, and it stinks!