Hang on. First off, by running fdisk to remake all your partitions, you'll be deleting all of your current data. Are you sure you want to do that? You'll also be deleting your current WinXP install.
If that's alright, here's what you have to do:
Boot to a command prompt from a Win98 install CD or boot disk.
Run fdisk.
Delete all of your current partitions.
Create a new Primary partition.
Create a new extended partition.
Create 3 logical partitions on that extended partition.
Reboot, and install Win98 on the primary partition.
Once installation is complete, reboot.
Boot from the WinXP CD.
Install WinXP on one of the other partitions.
When XP has finished installing, you will have a dual boot system.
But remember, this will everything you have now. There are other ways that won't delete everything, but this is the simplest.
-CC