pgsql: Update Windows installation notes.

Log Message:
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Update Windows installation notes.

pginstaller isn't used anymore, in favor of the one-click installers.
Make it clear that we support Windows 2000 and newer with the native
port, instead of first saying we support NT4 and then saying we don't.

Tags:
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REL8_4_STABLE

Modified Files:
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pgsql/doc/src/sgml:
installation.sgml (r1.325.2.4 -> r1.325.2.5)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/installation.sgml?r1=1.325.2.4&r2=1.325.2.5)