It seems that you are trying to install Windows on the Ubuntu partition without formatting the partition first.
Important: all the following steps will cause loss of data! Continue only if you really want to delete ALL the Ubuntu data (OS and your private data). You have been warned!
When installing Windows, format the partition from within the Windows installer, i.e. when installing. You should do it at the screens that look like the following:
If you cannot press the format button:
- Delete all the partitions (via the delete button).
- Make a new partition on the empty space (via the new button). Windows will tell you that it has to create a small system partition, this is normal.
- Format the new partition, be sure to format the bigger one and not the system partition created by Windows.
When you managed to format the partition, select that one to install to and you should be able to finish installing Windows.