I dont know if this will work, but here is a site explaining how to dual boot 98 and XP when XP is already installed. Might give you the general idea. Might not. Its worth a try though right?
Thanks go to sd for the link.
hxxp://www.dougknox.com/xp/tips/xp_repair_9x.htm
Remember, always back up everything before you do something outrageous like this.