sudo gem install rails not working because of problem with alias_method and is_file

I had downloaded ruby, rails, and rubygems previously on my hard drive
when I ran Windows. I just downloaded Ubuntu and tried to use RoR but it
wasn't working, so I figured I had to follow the installation procedure
listed here http://guides.rubyonrails.org/getting_started.html to get it
working. When I type in

sudo gem install rails

I get the following error message

/usr/local/lib/site_ruby/1.9.1/rubygems/package.rb:383:in
`alias_method': undefined method `is_file' for class
`Gem::package::TarReader::Entry' (NameError)
  from /usr/local/lib/site_ruby/1.9.1/rubygems/package.rb:383:in
`<class:Entry>'
  from /usr/local/lib/site_ruby/1.9.1/rubygems/package.rb:319:in
`<class:TarReader>'
  from /usr/local/lib/site_ruby/1.9.1/rubygems/package.rb:311:in
`<module:Package>'
  from /usr/local/lib/site_ruby/1.9.1/rubygems/package.rb:46:in
`<module:Gem>'
  from /usr/local/lib/site_ruby/1.9.1/rubygems/package.rb:18:in `<top
(required)>'
  from /usr/local/lib/site_ruby/1.9.1/rubygems/custom_require.rb:21:in
`require'
  from /usr/local/lib/site_ruby/1.9.1/rubygems/custom_require.rb:21:in
`require'
  from /usr/local/lib/site_ruby/1.9.1/rubygems/builder.rb:1:in `<top
(required)>'
  from /usr/local/lib/site_ruby/1.9.1/rubygems/custom_require.rb:21:in
`require'
  from /usr/local/lib/site_ruby/1.9.1/rubygems/custom_require.rb:21:in
`require'
  from /usr/local/lib/site_ruby/1.9.1/rubygems.rb:61:in `manage_gems'
  from /usr/bin/gem:4:in `<main>'

I have no idea how to solve this. At this point I just want to delete
everything having to do with rails and ruby and start over again. How do
I do this?

I strongly advise using rvm to install Ruby and Rails in Ubuntu

Colin