If you *absolutely* can not lose it then I would have to assume that you have a backup on tape or CD. If so make a backup, then restore onto your new OS (don't use a proprietary backup program that will not work on another OS).
You can't directly convert NTFS to any other filesystem, even other MS filesystems (at least to my knowlege). It would take a backup, reformat in whatever format you want then restore. You could always buy another hard drive and run two disks. Linux's NT filesystem code isn't the greatest though (the latest I knew you can only have read only access to NTFS partitions).
[ January 27, 2002: Message edited by: VoidMain ]