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:insend’
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:innew_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:innew_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