We know that there are a lot of pieces, and that's the main reason why we have the jbpm installer.
If the installer has failed because an external file of our control was moveds, the only thing that you need to do is to replace the link in the build.properties file for this one:
http://archive.eclipse.org/technology/epp/downloads/release/helios/SR2/eclipse-java-helios-SR2-win32.zip
Notice that this is recommended if you access to the old url, that you have pasted in the first message of the post:
BUILD FAILED
C:\jbpm-installer\build.xml:254: Can't get http://download.eclipse.org/technology/epp/downloads/release/helios/SR2/eclipse-java-helios-SR2-win32.zip to C:\jbpm-installer\lib\eclipse-java-helios-SR2-win32.zip
If you really need another version of the IDE, you can try the same approach, but in that case we cannot guarrantee that it will work.