[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:indefault_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:ininitialize’
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:inpaths’
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:indirs’
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:ineach’
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:infind_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:inrequire’
from C:/RailsInstaller/Ruby1.9.3/bin/gem:9:in `’

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