If your system only boots into memtest and there’s no entry in Grub for ubuntu after Ubuntu Karmic installed,this post may help!
Start your machine and after BIOS screen, keep SHIFT key pressed. GRUB menu should be there. If you can see the appropriate entry to boot, try it. It may work.
If there’s only memtest entry or other entries,press c for a GRUB2 command line.There should come the GRUB shell.

sh:grub> _

Following is about my partitions,ubuntu karmic was installed in /dev/sda6 and the installation disk in /dev/sda3:

/dev/sda1 : Primary, VFAT.
/dev/sda3 : Primary, ext3. My installation disk area (~760MB)
/dev/sda2 : Extended
|__ /dev/sda5 : swap.
|__ /dev/sda6 : ext4. My main installation area (20GB)

1.Try to boot into my new disk using casper kernel, to reinstall linux-image to solve problem.

sh:grub> set root=(hd0,3)
sh:grub> linux /casper/vmlinuz root=/dev/sda6 rw
sh:grub> initrd /casper/initrd.lz
sh:grub> boot

Note:If you install Ubuntu Karmic from Windows system and assume the Windows partition is /dev/sdax (vmlinuz and initrd.lz are in the root directory of C:),use following instead previous codes:

grub>set root=(hd0,x)
grub>linux /vmlinuz root=/dev/sda6 rw
grub>initrd /initrd.lz

2.Now it should boot into your newly installed system using LiveCD’s kernel.Open terminal and reinstall linux-image:

sudo apt-get clean
 sudo apt-get update
 sudo apt-get install --reinstall linux-image-'uname -r'

3.Finally,reconfigure your GRUB2:

grub-install /dev/sda
sudo update-grub

Hope this help!
Original text:http://ubuntuforums.org/showthread.php?t=1319202
Merry Christmas!