The Eclipse executable launcher was unable to locate its companion launcher jar windows

The most common reason for this message seems to be unzipping the eclipse zip file wrongly (for instance unzipping without recreating the directory structure). Therefore please unzip the zipped Eclipse again with a good unzip tool (like 7-zip) and make sure that the necessary sub directories are created during the extraction.

Also make sure that the path to the unzipped Eclipse does not get very long. I've seen cases where Eclipse was unzipped into a deeply nested directory structure (to put it at some place into an SVN repository) and that led to the same error message.

If that still doesn't work, you may try launching eclipse.exe with administrative rights. That should not really be necessary, but maybe your access rights are somehow broken after the re-installation of Windows.

Edit the eclipse.ini file and remove these two lines:

-startup
plugins\org.eclipse.equinox.launcher_1.0.100.v20080509-1800.jar 

Same issue on my Mac with a pre-packaged RCP client that was unzip only. The other answers pointed me to the eclipse.ini where all the paths looked valid, tried to check the working directory of my Eclipse bundle that doesn't see that easy to do. Seems when Finder starts an application it's working directory is / so that seems to be the issue.

Updated both references in my eclipse.ini to absolute paths and voila!

-startup
/Applications/RCPClient/client/eclipse/plugins/org.eclipse.equinox.launcher_1.1.1.R36x_v20101122_1400.jar
--launcher.library
/Applications/RCPClient/eclipse/plugins/org.eclipse.equinox.launcher.cocoa.macosx.x86_64_1.1.2.R36x_v20101019_1345
...