OK,
Download a 98 boot disk from here, when its downloaded,pop in a blank floppy and run the .exe.This will write all the boot files to the floppy.

Now start the 98 machine and put the boot floppy in,choose boot into command prompt with CD-ROM support.You should now get an A> prompt.(if you don't get the A> prompt,type in A: )

At the A:\> prompt type in format c: (This will wipe all data from the C: drive.)

When the format is complete pop in the XP CD and type d:\:setup and hit enter. The XP setup program should now run.