I think the only way you could set up dual boot with XP already installed would be to use a 3rd party boot manager.
No, there is another way:
First you want to format the hard drive(s). If you only have one hard drive than you want to split the hard drive into two partitions. One drive or partition must be formated Fat32 and the other drive or partition formated in NTFS. Then you want to install Win98 first on to the Fat32 drive or partition. After Win98 is installed then install WinXP on to the NTFS drive or partition.
After all of that is done then do the following:
In WinXP go into the Control Panel and double click Systems. Click the Advanced Tab then click Settings in the Startup and Recovery subsection. In the System startup subsection under the Default operating system click the operating system you want the boot up to default to. Then put a check in the box forDisplay list of operating systems and then to the right give yourself some time to make that choice.
IMHO this option is a little tricky and can cause problems from what I've been told. What I did when I dual booted in Win2000/WinXP Pro is installed each operating systems on to different hard drives and then when I wanted to switch to the other operating system I rebooted and changed the boot order in Bios. This way took a little longer but insured that there wasn't any problems. Hope this helps.