The short answer is: it depends. Assuming that you have a single physical disk with multiple partitions, what will happen when you install windows is that it will override the master boot record. So, in order to boot GNU/Linux the easy way, you will want to restore the boot loader.
The--probably--easiest way is to have some bootable media (floopy or Ubuntu live CD) which will allow you to boot some GNU/Linux system. From there, chroot into your "real" filesystem root, and run grub-install as root.
Make sure to read the grub manual first, and try "practicing" by installing grub on, say, a floppy.