Most likely to get NT back on you're going to have to start all over and FDISK(MSDOS) and format everything for NT to install. Windows 2000 never seemed to have a problem with my Ext2, ReiserFS, NTFS(5), and FAT16 partitions on the same drive, but if you alreay have NT4(?) then stick with it.
That's the best solution that I can think of...but if you're daring, I think this will fool NT. By using the FDISK on the RedHat 6.2 CD(It's prolly in other places too) you can change the ID of the partions to be FAT16 so that it's not mad anymore about 'foreign' partitions. Just don't let it format them! Then after NT is done installing change them back to the old ID (write it down!) and everything should be set.