It depend on if you are using the wubi installer (which installs ubuntu...

It depend on if you are using the wubi installer (which installs ubuntu inside of windows) or installed ubuntu on another partition.

If you installed ubuntu on another partition then you can delete the old windows partition using a tool like gparted and create a new partition to use with ubuntu.

If you installed ubuntu inside of the windows partition using the wubi installer then you have a bigger fight ahead of you, you would have to backup all of the data you wish to preserve on an external drive or disk, then boot into the ubuntu installation disk (cd or DVD) to run a entire disk installation, once that is complete you can transfer your files back into ubuntu.

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