Strange the installer didn't detect it!
Try to add an entry in /boot/grub/menu.lst for Windows manually, e.g.:
This is if Windows is located on the first partition of the first contoller. If you have Windows on another partition or even another HDD, you need to adjust (hdx,y) accordingly. In the second case it may need to look like this
map (hd0) (hd1)
map (hd1) (hd0)
in order to trick Windows to believe it's located on the first drive.