I installed Ubuntu OS. Then tried to install Windows 7 as dual boot. But i get error message that unknown file system. Not NTFS. Now there is no OS in System. How to change the hard drive file system to NTFS to install Windows 7 and then Ubuntu?
You can do it from any installation media (ubuntu/windows 7).
Take a backup important data using any live OS (like ubuntu). Following procedure will delete the data in your system. You can not change file system type without deleting data.
Since you want to install windows 7 first, do the following steps..