My current working grub.conf is:
default=0
timeout=10
splashimage=(hd0,4)/grub/splash.xpm.gz
title Fedora Core (2.6.5-1.358)
root (hd0,4)
kernel /vmlinuz-2.6.5-1.358 ro root=LABEL=/ rhgb quiet
initrd /initrd-2.6.5-1.358.img
title Windows
rootnoverify (hd0,0)
chainloader +1

Besides the Fedora Core 2 and windows installations on the hard disk, I had installed linux on the hard disk previously but I forgot what are the distributions.
How do I find out what linux distributions are installed on the hard disk?
How to modify grub.conf so that I can boot into those linux distributions?

# fdisk /dev/hda

Disk /dev/hda: 80.0 GB, 80000000000 bytes
255 heads, 63 sectors/track, 9726 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Start End Blocks Id System
/dev/hda1 * 1 3213 25808391 7 HPFS/NTFS
/dev/hda2 4519 9726 41833260 f W95 Ext'd (LBA)
/dev/hda3 3214 3342 1036192+ 82 Linux swap
/dev/hda4 3343 4518 9446220 83 Linux
/dev/hda5 4519 4534 128488+ 83 Linux
/dev/hda6 4535 9726 41704708+ 83 Linux

Try mounting (su, cd /mnt, mkdir hdaX, mount /dev/hdaX ./hdaX) those other partitions marked as 'Linux' (hda4, hda5, hda6), and seeing what's on them to find what else is installed. Adding them to your GRUB menu is more complex, and I would give some advice, but I haven't done it much myself.

I remember hda6 has a FC2 installation.
Now I installed FC3 on hda4.
How to modify the grub.conf to allow booting to FC2?

grub.conf is:
default=0
timeout=5
splashimage=(hd0,3)/boot/grub/splash.xpm.gz
hiddenmenu
title Fedora Core 3 (2.6.9-1.667)
root (hd0,3)
kernel /boot/vmlinuz-2.6.9-1.667 ro root=LABEL=/1 rhgb quiet
initrd /boot/initrd-2.6.9-1.667.img
title windows
rootnoverify (hd0,0)
chainloader +1



# ls /boot
config-2.6.9-1.667 initrd-2.6.9-1.667.img vmlinuz-2.6.9-1.667
grub System.map-2.6.9-1.667

# cd /mnt
# mount /dev/hda6 hda6

No files in /mnt/hda6/boot directory?
# ls hda6/boot/