Which boot loader do you use? Grub or lilo? I suppose it's Grub, so boot your Fedora system and edit /boot/grub/menu.lst. You'll find stanzas like this there:
title Xen 2.0.6 / XenLinux 18.104.22.168-xen0
kernel /xen.gz dom0_mem=65536
module /vmlinuz-22.214.171.124-xen0 root=/dev/hda6 ro console=tty0
The system that is listed first is the default one. So if you change the order so that your Windows system is the first one it will be booted by default.