I recently wanted to try Ubuntu 12.04 on my laptop (Dell mini 10). I messed around quite a bit and ended up destroying some vital part of the mbr or the partition table. I finally fixed the partition error by wiping and reinstalling. I copied all my data first and this time I went for Ubuntu 11.04 (minimal install) which I had earlier. After getting it installed I had a strange problem–it would only boot if the usb drive was plugged in.
I thought that maybe it had copied the grub2 information to the usb instead of the harddrive. What I did to correct the problem was simple. I booted up, with the usb plugged in then I typed:
sudo grub-install /dev/sda
Then unplugged the usb and rebooted I was good to go.
Other Links:
https://help.ubuntu.com/community/RecoveringUbuntuAfterInstallingWindows