i had dual booting machine..with windows xp and ubuntu...and 15gb of free HDD space
i decided to load fedora in the free space..
my ubuntu partitions are stated below ->

/boot 256 MB /dev/hda9
/ 12 GB /dev/hda10
SWAP 768 MB /dev/hda11

i installed fedora succesfuuly in the free free space..
it had partittions as stated below ->

/boot 256 MB /dev/hda12
/ 14 GB /dev/hda13
SWAP 768 MB /dev/hda14 (now,i think this swap partition was unnecessary..as i had already one swap partition functioning)

While installing fedora ,in the boot loader configuration setup,
there were booting options such as fedora and others (windows)..
but no entry for ubuntu..
hence i added ubuntu and gave /dev/hda10(root partition of ubuntu)
as path for ubuntu in the boot loader..
next..when i try to boot ubuntu in GRUB the following message shows up :

< booting 'ubuntu'
rootnoverify(hd0,9)
chainloader +1
invalid or unsupported executable format >

i have surely messed up while installation.
at the boot loader configuration setup what should i am supposed to give the path of ubuntu????should that be /boot(/dev/hda9) instead of root partition(/dev/hda10)

now..is there any way to fix this problem??

From the Grub Documentation Page:

Invalid or unsupported executable format
This error is returned if the kernel image being loaded is not recognized as Multiboot or one of the supported native formats (Linux zImage or bzImage, FreeBSD, or NetBSD

Fedora doesn't do well at detecting or making entries for other OS's in the grub.conf (menu.lst) file. You need to make a manual entry. I assume you are able to boot Fedora? If so, just mount Ubuntu in Fedora and go to the Ubuntu /boot/grub/menu.lst file and copy the Ubuntu entry from there to /boot/grub/grub.conf in Fedora. If you don't know how to do this, post again. Also, it would be useful to post the output of 'fdisk -l' (lower case letter L).

Do you have two boot partitions on your drive (hda9 and hda12)??