Hi,,
if you want to ttriple boot then,you may need to install the grub again.Whrn you install windows it overwrites grub.
Now you boot with RedHat cd/dvd--->Go to Rescue mode--->do
Code:
chroot /mnt/sysimage
fdisk -l
grub-install </dev/sda or /dev/hda)>
Then open file
and make the entry of Windows 2008 Server in it