you need to edit /boot/grub/menu.lst
change the "default" option
Code:
## default num
# Set the default entry to the entry number NUM. Numbering starts from 0, and
# the entry number 0 is the default if the command is not used.
#
# You can specify 'saved' instead of a number. In this case, the default entry
# is the entry saved with the command 'savedefault'.
default 1
so if windows is second on your list change default to 1