If you are using vagrant, you probably-statistically are using git. Make sure you have its binary folder on your path, because that path contains 'ssh.exe'.
Now, modify C:\vagrant\vagrant\embedded\lib\ruby\gems\1.9.1\gems\vagrant-1.0.3\lib\vagrant\ssh.rb to comment out the faulty Windows check and add a real SSH check:
# if Util::Platform.windows?
  # raise Errors::SSHUnavailableWindows, :host => ssh_info[:host],
                                       # :port => ssh_info[:port],
                                       # :username => ssh_info[:username],
 # :key_path => ssh_info[:private_key_path]