Rails - Oct 2012 - [Win7 x64] Encoding::UndefinedConversionError after installing when using gem and rails

Hello,

after installing the RailsInstaller i got the following error:

C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems/defaults.rb:57:in 
`exist?'': "\x81" to UTF-8 in conversion from Windows-1252 to
 UTF-8 (Encoding::UndefinedConversionError)
        from 
C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems/defaults.rb:57:in 
`default_path''
        from 
C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems/path_support.rb:63:in
`path=''
        from 
C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems/path_support.rb:29:in
`initialize''
        from 
C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems.rb:392:in 
`new''
        from 
C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems.rb:392:in 
`paths''
        from 
C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems.rb:413:in 
`path''
        from 
C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems/specification.rb:382:in
`dirs''
        from 
C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems/specification.rb:267:in
`_all''
        from 
C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems/specification.rb:410:in
`each''
        from 
C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems.rb:480:in 
`map''
        from 
C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems.rb:480:in 
`find_files''
        from 
C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems.rb:1085:in 
`load_plugins''
        from 
C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems/gem_runner.rb:86:in
`<top (required)>''
        from 
C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in
`require''
        from 
C:/RailsInstaller/Ruby1.9.3/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in
`require''
        from C:/RailsInstaller/Ruby1.9.3/bin/gem:9:in `<main>''

I get this error every time when i execute gem -v or rails -v and so on. 
The command ruby -v works!

I have a Windows 7 64bit machine and used the current version of the rails 
installer.
Can someone help?

Greedings,
Malte

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