I guess the 15 GB is the disk space in which ubuntu is installed . Do you want to keep ubuntu or not ?
Download Partition Wizard Bootable CD
, burn it in a disk . Boot from the PW disk , you will see the rest as unpartitioned space . Format it as NTFS primary partition (if you want to keep ubuntu, otherwise you may format the entire disk as NTFS primary using partition wizard ) .
Now you can install windows .