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Don't use them. 20.x will not work, 21.0.x were prereleases, anyway.
As of GNU Emacs 21.2, no image support is available in Emacs under Windows. Without images, preview-latex is useless. The current CVS version of Emacs available from http://savannah.gnu.org/projects/emacs now supports images including the PNG format, so Emacs 22 should work out of the box once it is released. Another option for Windows users might be to try XEmacs. preview-latex has successfully been installed with recent CVS versions of GNU Emacs, and both Cygwin XEmacs and the native Windows XEmacs.
See Installation under MS Windows for detailed installation instructions for this platform.
There is are two larger problems known with older XEmacs releases. One leads to seriously mispositioned baselines and previews hanging far above other text on the same line. This should be fixed as of XEmacs-21.4.9.
The other core bug causes a huge delay when XEmacs's idea of the state of processes (like ghostscript) is wrong, and can lead to nasty spurious error messages. It should be fixed in version 21.4.8.
Previews will only remain from one session to the next if you have version 1.81 or above of the edit-utils package, first released in the 2002-03-12 sumo tarball.