Grub Error 15 : File not found
This error is returned if the specified file name cannot be found,
but everything else (like the disk/partition info) is OK.
The disk and partition information in the grub configuration are OK, but the file grub is looking for (either the kernel image or the initrd image) is/are incorrect.
To correct this, we need to see the Ubuntu menu entry in /boot/grub/menu.lst, and the contents of the /boot directory.
Boot Ubuntu from the cd. Mount the partition in which you installed Ubuntu. Assuming it's the first partition of the first drive, the Ubuntu running from the cd would see it at /dev/sda1 with a mount point of /mnt/sda1. The command to mount the partition, in a console, is 'mount /dev/sda1 /mnt/sda1' (without quotes).
Then cd to /mnt/sda1/grub. Give the command: sudo cat menu.lst. Copy (write it on paper if you need to) the Ubuntu menu entry that starts with 'title Ubuntu'. It's probably three lines: title Ubuntu, kernen ....., initrd .... Write them down exactly as they appear.
Then give the command: ls /mnt/sda1/boot. Write down the filenames for vmlinuz and initrd exactly as they appear. If there is a difference between what shows in /boot and what's in the file /boot/grub/menu.lst, then correct menu.lst to agree with the filenames in /boot.
To edit /boot/grub/menu.lst, you must have root authority, so use the command: sudo gedit /boot/grub/menu.lst to open the file with gedit. Then make your corrections and save the file,
Then try a reboot to see if it worked. If not, write down any errors and report them here.
Note: I'm assuming that you installed Ubuntu with the gnome desktop environment and are using gedit as the default editor.