This can be done easily in Ubuntu.
First use the Ubuntu live CD and boot your system.
Then locate the root.disk file. which is your Ubuntu's root directory.
If you have installed Ubuntu inside Windows
you may find this in C:\ubuntu\root.disk
in such situation you need to first mount your filesystem partition. This can be done by
sudo mkdir /win
sudo mount /dev/sda1 /win
always, replace "sda1" with the appropriate configuration, i.e. "sda2" or "sda3"...
(just remember: a=disk, 1=partition number)
and now you can find your partition in /win Directory.
Now you can mount your root.disk and this can be done by:
sudo mkdir /vdisk
sudo -o loop /win/ubuntu/disks/root.disk /vdisk
now your files in root.disk can be accessed through /vdisk