That's easy! Look in /boot/grub for a file called menu.lst or possibly grub.conf. Open up the file and find the line:
Next, look further down the file for the entries for the two operating system. It will look similar to this:
kernel /boot/kernel root=/dev/sda1 vga=0x318 video=vesafb
Change the number after "default" to match the position that the Windows entry appears at (positions start at the number zero). It will probably be the second entry so change the number to "1". I hope that was clear enough.