Re: ubuntu overwrites the whole partition?
looks like you try to install via Wubi. Wubi installs only creates a virtual partition inside Windows. This is for people that want to try out Ubuntu to an extent that a Live CD session cannot provide. So in other words, a Wubi install is not a real ubuntu insallation. To make a full installation you would need to boot from a Live Cd, create a new ext4 filesystem partition where Ubuntu would be installed to.
And, to answer your original question, no, it would not simply erase your partition, but instead, take some space and create a virtual partition.
The advantage of a wubi install is that you can easily uninstall it if you do not like Ubuntu. The drawback is that if you can't boot into windows, you also have no access to Ubuntu.
Hope this helps
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