yes , as i already wrote, i've tried to rebbot eclipse and even do a clean installation - removing entire folder of both eclipse and the workspace . they both failed.
downloading succeeds , but while it tries to install , it gives me this error and then quits the installation process .
btw, i use the next string for the installation (via the "install new software" menuitem of eclipse) : http://dl.google.com/eclipse/inst/d2wbpro/latest/3.6
EDIT:after cleaning the computer from temporary files , i've tried to re-install window builder, and it seems to work .
it's weird. maybe i didn't have enough space on the HD or something.