If you want to install Windows, you have to have a spare partition. And that's all there is to it. No matter if you have Linux already on the harddisk. Just free one partition and install Windows on it. Remebere Windows requires a primary partition to install to. And will sometimes complain about not having the first partition. Once you install Windows, you will lose the GRUB boot loader and will have to reinstall it. Get the FC3 rescue disc and boot from that. Once inside, you have to run
grub> root (hd0,0)
grub> setup (hd0)
$ /sbin/grub-install /dev/hda
and logout to reboot. You should have a grub boot loader up and running again.
Replace hd0,0 and hda according to your partitions. Any other questions you might have, post them here.