ok this is straight outa the MSCE book on how to ditch NT and keep windows 98 intact.
you are gonna need a win98 boot disk. make one from ctrl panel | addremove programs | startup disk.
boot computer from the win98 boot floppy and when you get to the a:\ prompt type "sys c:" without the ", this will get rid of the boot menu that you have asking weather you want to boot into NT or win98. from there, you can now boot into win 98 and all you need to do is delete the NT directory entirely.
NT is now gone.
-that is if you aren't using NTFS for your NT operating system. (if you can see your NT files while booted into win98, you aren't running NTFS.) if that's the case, after you have run the sys program, run fdisk and delete the NTFS partition (will show up as a non DOS partition) and you will be NT free.
good luck.