As Daniel says, you simply can't. You MUST do a clean install of ME (or
some other system).
If you installed W2k by attempting an upgrade of a previously installed
ME, are dissatisfied with the results, and for that reason want to get
rid of W2k and revert to ME, you might try a clean install of W2k
instead. The unsupported "upgrade" path from ME to W2k is a recipe for
disaster almost always, with many lingering problems and very erratic
W2k behavior. It's an awful way to experience W2k, which when properly
installed is an extremely stable, robust system.
Try to save all the data you can before formatting that OS partition.
You'll have to reinstall all your apps no matter what you do, but data
files can be backed up.