I don't know if current installers will detect the windows partition and
add it to the new grub menu or not. If it doesn't you will be able to
boot into Linux no problem and then just have to edit /boot/grub/menu.lst
to have the entry for windows. If it is working now perhaps make a deadtree
copy of the menu.lst file so you know what the windows entry looks like
if you have to restore it at some point.