Essentially, Windows 98 is from the 9x series of Windowses. Since they are basically DOS with a GUI, they can thus be booted from a DOS environment.
Windows 2000, XP, Vista, etc. are from the NT series, which are NOT based on DOS. They cannot be booted from a DOS environment. Since ReactOS is based on the designs of the NT series, it cannot be booted from a DOS-like OS.
You can, however, make a floppy-based boot-disk if you cannot boot directly from an install CD. See this post I made a while ago: viewtopic.php?f=31&t=5755&p=46894#p46894