JDK Tested: 1.5.0_06 erdist 20051219
Launch htmlconverter as follows -

$ jdk1.5.0_06/bin/java -jar jdk1.5.0_06/lib/htmlconverter.jar -gui

The following is displayed instead of the gui -

/Usage: HtmlConverter [-option1 value1 [-option2 value2 [...]]]
[-simulate] [filespecs]

where options include:

-source: Path to get original files. Default: <userdir>
-source -: read converting file from the standard input
-dest: Path to write converted files. Default: <userdir>
-dest -: write converted file to the standard output
-backup: Path to write backup files. Default: <dirname>_BAK
-f: Force overwrite backup files.
-subdirs: Should files in subdirectories be processed.
-template: Path to template file. Use default if unsure.
-log: Path to write log. If not provided, no log is written.
-progress: Display progress while converting. Default: false
-simulate: Display the specifics to the conversion without converting.
-latest: Use the latest JRE supporting the release mimetype.
-gui: Display the graphical user interface for the converter.

filespecs: Space delimited list of files specs. Default: "*.html *.htm"
(quotes required)/

The htmlconverter gui lauches ok using 5.0u6FCS [5.0u6b5]