Do what I did to my wife: remove XP from the boot loader. After 3 months, I put it back and she refuses to use Windows.
Or, as root, edit /boot/grub/menu.lst. Find the line that says "Default X" where X is probably 0 or 1. Whatever number XP is in the boot list, change the "Default" to be one less. If XP is 3rd on the boot menu, change it to "Default 2".