Could not find buildr (>= 0)

[Ittay Dror]

I have Windows 7

I installed buildr according to instructions, but when I try to use it I 
get:

> buildr
C:/Ruby187/lib/ruby/site_ruby/1.8/rubygems/dependency.rb:247:in 
`to_specs': Could not find buildr (>= 0) amongst [atoulme-Antwrap-0.7.1, 
builder-2.1.2, buildr-1.4.6-x86-mswin32, diff-lcs-1.1.2, highline-1.5.1, 
hoe-2.3.3, json_pure-1.4.3, minitar-0.5.3, net-sftp-2.0.4, 
net-ssh-2.0.23, rake-0.8.7, rjb-1.3.2-x86-mswin32-60, rspec-2.1.0, 
rspec-core-2.1.0, rspec-expectations-2.1.0, rspec-mocks-2.1.0, 
rubyforge-2.0.3, rubygems-update-1.8.5, rubyzip-0.9.4, 
xml-simple-1.0.12] (Gem::LoadError)
        from 
C:/Ruby187/lib/ruby/site_ruby/1.8/rubygems/dependency.rb:256:in `to_spec'
        from C:/Ruby187/lib/ruby/site_ruby/1.8/rubygems.rb:1182:in `gem'
        from C:/Ruby187/bin/buildr:18

How can I fix this?

Thanks,
Ittay