there are many things that can go wrong with this method, so i wouldnt suggest it unless u knew what you were doing and are willing to take the risks.
go to
www.bootdisk.com and downoad a windows98 bootdisk (this is assuming that you are on a fat32 partition).
put the disk in and then reboot the pc.
it should take you to a dos prompt.
now, do a "format c:"
this will erase EVERYTHING from the C drive.
now, pop in the cdrom drive and then boot up into the windows ME install.