How to reinstall Ubuntu as part of dual boot?
Solution 1:
It is rather easy to do ;-)
Start your system with your Ubuntu Live CD in the CD bay. After it loaded you choose install at this image:
and at some point it asks for where to put the installation in a screen similar to this (with options related to your system):
Here you can choose for erase Ubuntu and reinstall or the something else option to get done what you want. Be careful though: The erase everything will also delete Windows and if you do choose something else and mess up by choosing the wrong lay-out you could potentially also remove Windows.