Ubuntu – WUBI installed Ubuntu not showing up at boot menu after Windows reinstallation

For XP, right click My Computer, click on Properties, click the Advanced tab, Startup & Recovery settings.

Make sure that the Time to display operating systems is 10.

If the timeout is not zero, then in rare cases there is a line missing from the end of the C:\boot.ini required to boot Ubuntu: C:\wubildr.mbr = "Ubuntu"

If you decide to edit the boot.ini, make sure you back it up, and edit it from the same place.

It should look something like this:

[boot loader] 
timeout=10 
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS 
[operating systems] 
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect 
C:\wubildr.mbr = "Ubuntu" 

For Windows 7, right click on Computer, Properties, Advanced system settings (on the left):

Then click on Startup & Recovery settings:

Make sure the Time to display list of operations systems is set to 10 or above. If that's not the problem, run CMD.EXE selecting to Run as an administrator, then enter bcdedit to make sure there is an entry for Ubuntu.

If You have a 12.04 Live CD or bootable USB, Boot using that to Live Mode.

Then mount your 'root.disc' and 'oldroot.disc' files in two folders in Live users home folder. (I'm assuming your two files as root.disc and oldroot.disc, You'll use the exact name for them in the commands)

First make two folders

mkdir newroot oldroot

Then mount them accordingly

sudo mount -o loop root.disc ~/newroot 

sudo mount -o loop oldroot.disc ~/oldroot

Then copy all files from newroot's boot to oldroot's boot

sudo cp -r ~/newroot/boot ~/oldroot/ 

After copy is finished, unmount both of the files.

sudo umount ~/newroot
sudo umount ~/oldroot 

Exit from live session. Make necessary adjustment to use the oldroot.disc file such as renaming. After that start your Ubuntu with the hope it is fixed.