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GRUB error 25 means that: "This error is returned if an unrecognized
command is entered into the command-line or in a boot sequence section
of a config file and that entry is selected"

You can follow the instructions posted at Ubuntuforums.org:

Boot with your Ubuntu installation CD
On the boot: prompt, type rescue and hit enter
Follow the instructions on the screen
When the computer is ready, assuming that /dev/hda is where your /boot
is located, type: grub-install /dev/hda.

the seperate /home partition is where all your own files are located,
like for example, your user account settings for the Pidgin chat
program, or your firefox bookmarks. So if you keep your /home
partition separate from your / partition, you can reinstall ubuntu,
and have your settings preserved.


-- 
Saad M Niamatullah