problem with script/plugin install

Whenever I try to install a plugin I get the following error:

e:/apps/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.1/lib/ active_support/dependencies.rb:422:in `remove_const': cannot remove Object::OPT_TABLE (NameError)         from e:/apps/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.1/ lib/active_support/dependencies.rb:422:in `send'         from e:/apps/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.1/ lib/active_support/dependencies.rb:422:in `remove_c onstant'         from e:/apps/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.1/ lib/active_support/dependencies.rb:368:in `new_cons tants_in'         from e:/apps/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.1/ lib/active_support/dependencies.rb:368:in `each'         from e:/apps/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.1/ lib/active_support/dependencies.rb:368:in `new_cons tants_in'         from e:/apps/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.1/ lib/active_support/dependencies.rb:495:in `require'

        from script/plugin:3

I would be grateful if somebody could shed some light. Thanks George