Converting your fat to ntfs will not damage any data in the hard disk(but converting it may make it unreadable under dos)
First open the dos prompt and type the command exactly as follows:
convert e: /fs:ntfs
This will convert your e drive to ntfs file system .
If you want to convert a particular drive then type the drive alphabet in place of e
But sometimes the system might ask for a restart so give ok.
Ntfs file systems are used whenever you have to encrypt the files in your hard disk.
They are also required to run some games.