gulp-imagemin and gulp-ruby-sass not working in windows 7

We are about to integrate gulp.js in our workflow. Until now we used compass to compile our scss files into css. I installed gulp according to documentation and can't get gulp-ruby-sass and gulp-imagemin to work.

Here is my gulpfile.js

var gulp = require('gulp'),
    gulpLoadPlugins = require('gulp-load-plugins'),
    uglify = require('gulp-uglify'),
    cache = require('gulp-cache'),
    rename = require('gulp-rename'),
    imagemin = require('gulp-imagemin'),
    pngcrush = require('imagemin-pngcrush'),
    minifyCss = require('gulp-minify-css'),
    rubySass = require('gulp-ruby-sass'),
    concat = require('gulp-concat'),
    autoprefixer = require('gulp-autoprefixer'),
    plugins = gulpLoadPlugins();

// styles
gulp.task('sass', function () {
    return gulp.src('sass/styles.scss')
        .pipe(plugins.rubySass({ style: 'compressed' }))
        .pipe(plugins.autoprefixer('last 15 versions'))
        .pipe(plugins.minifyCss())
        .pipe(gulp.dest('styles'));
});
// scripts
gulp.task('scripts', function () {
    return gulp.src('javascript/*.js')
        .pipe(uglify())
        .pipe(plugins.concat('general.min.js'))
        .pipe(gulp.dest('scripts'));
});
// images
gulp.task('images', function () {
    return gulp.src('images/source/*')
        .pipe(plugins.cache(plugins.imagemin({ optimizationLevel: 3, progressive: true, interlaced: true })))
        .pipe(gulp.dest('images'));
});
// watch files for changes
gulp.task('watch', function () {
    gulp.watch('sass/*.scss', ['sass']);
    gulp.watch('javascript/*.js', ['scripts']);
    gulp.watch('Images/Source/**/*', ['images']);
});
// default task
gulp.task('default', ['watch']);

Here is my package.json

{
    "name": "test1",
    "version": "1.0.1",
    "dependencies": {
        "gulp": "^3.8.7",
        "gulp-autoprefixer": "0.0.10",
        "gulp-cache": "^0.2.1",
        "gulp-compass": "^1.3.1",
        "gulp-concat": "^2.3.4",
        "gulp-load-plugins": "^0.6.0",
        "gulp-minify-css": "^0.3.7",
        "gulp-rename": "^1.2.0",
        "gulp-ruby-sass": "*",
        "gulp-uglify": "^1.0.0",
        "imagemin-pngcrush":"*",
        "gulp-imagemin":"*"
    }
}

Here are my errors: When I run "gulp sass"

[13:43:55] Starting 'sass'...

events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: spawn ENOENT: Missing the Sass executable. Please install and make availa
ble on your PATH.

I have Compass and Sass installed (and reinstalled).

When I run "gulp images"

[13:57:46] Starting 'images'...
Possibly unhandled Error: spawn ENOENT
    at errnoException (child_process.js:1001:11)
    at Process.ChildProcess._handle.onexit (child_process.js:792:34)

Additional information: I reinstalled ruby, nodejs, gulp, compass, sass and all the plugins for gulp. I searched google for Error-Messages and infos, but I couldn't come up with a fix for my problems. I also tried gulp-sass (not gulp-ruby-sass) and it works, but I need ruby-sass (because the team I work in uses ruby-sass).