“Ubuntu 11.04 error, “prefix” is not set” while booting Ubuntu
What drive did you use to install Ubuntu? Is it on the C: drive?
If you used any other removable drive, make sure your BIOS sees the appropriate drive.
After making sure your BIOS sees that drive, you need to find the directory C:ubuntu
It's possible that the root.disk is corrupted.
If you find the file, check whether says 0 bytes.
If it does, then you need to replace the file with a working one.
Do you have a back-up of your Ubuntu install?
If you have, you can get the previous root.disk file and replace the bad one in the directory where you found it.
This should fix your root.disk and enable you to boot in Ubuntu.
I think you have successfully installed Linux Ubuntu on your computer using Wubi and at first I thought you are installing Linux Ubuntu on a freshly formatted computer until I saw Wubi in your post. I then realized that there is another operating system running on your computer. Wubi is the official installer used for installing Linux Ubuntu on a computer running on Microsoft Windows. Wubi allows the easy installation and removal of Linux Ubuntu as if it was just an ordinary application that can be installed and uninstalled.
But remember that Wubi doesn’t work on a computer that uses the UEFI firmware or has the Windows 8 logo. But if in case you fall in this situation, use the 64-bit flavor of Linux Ubuntu and install it directly to the partition instead of using Wubi. To download the latest version of Wubi, visit Download Windows installer for Ubuntu Desktop. Installing Linux Ubuntu using Wubi is very simple. First, download the latest installer from the link above and then run it on your computer. Insert your password for the new account, change the settings like the disk space and others, and then click Install.
The whole installation is fully automatic. This is the screenshot of the Wubi installation screen when installing Linux Ubuntu.
After successfully installing Linux Ubuntu using Wubi, you can use the command prompt as an alternative to shutdown Linux Ubuntu.
I think the command you used to shutdown Linux Ubuntu is incorrect. To shutdown Linux Ubuntu from the command prompt, enter the command without quotes “sudo halt” or “sudo shutdown -h now” and then press Enter. On the other hand, to reboot Linux Ubuntu use the command without quotes “sudo reboot”.
To know the complete functions of reboot and shutdown, use the commands “man reboot” and or “man shutdown” to display their respective information.