Install Windows XP from the hard drive with Windows 98 already installed:

Boot Windows 98
Insert the XP CD into your CD
Explore Windows XP through My Computer
Copy i386 folder to C:
Go into C:i386 folder and double click on winnt32.exe to launch the setup from the hard drive

Install Windows XP from DOS (ie. no OS on a new hard drive):

Boot with a Windows 98 Start Up disk
Insert the Windows 98 CD
Run smartdrv.exe from the Win98 directory on the windows 98 CD
Type cd.
Insert Windows XP CD
Copy the i386 folder to C:
Go into C:i386 folder on C: and type winnt.exe to launch the setup from the hard drive.