Defrag trouble in FAT32

My son's PC is having a FAT32 hard disk with 80GB capacity. 70 GB has been used up and the drive has not been defragmented for years. Needless to say a slow access time is found. I tried to use the defrag program of Windows with a hope to reduce scatter fragment but Windows prompted me that at least 15 % of HD space should be available. Shit ! In a 80 GB HD, 12 GB will be sacrified due to poor design of FAT32. This leaves me no choice but to convert the whole HD from FAT32 to NTFS.