Wordpress - Display posts of the last 7 days

In addition to birgire's solution, as of WordPress 3.7, you can use Date parameters.

Your arguments would look like this to filter posts from the last 7 days:

$args = array(
    'post_type' => 'post',
    'post_status' => 'publish',
    'orderby' => 'date',
    'order' => 'DESC',

    // Using the date_query to filter posts from last week
    'date_query' => array(
        array(
            'after' => '1 week ago'
        )
    )
); 

I think this must have been solved many times here on WordPress Answers.

You could also check out the examples in the Time parameters part in Codex for WP_Query.

Here are two of them (slightly modified to your needs)

Example 1:

// Create a new filtering function that will add our where clause to the query
function filter_where( $where = '' ) {
    // posts in the last 7 days
    $where .= " AND post_date > '" . date('Y-m-d', strtotime('-7 days')) . "'";
    return $where;
}

add_filter( 'posts_where', 'filter_where' );
$slider_query = new WP_Query('posts_per_page=5&cat=3&orderby=highest_rated&order=desc');    
remove_filter( 'posts_where', 'filter_where' );

Example 2:

// Create a new filtering function that will add our where clause to the query
function filter_where( $where = '' ) {
    // posts for May 1 to March 8, 2013
    $where .= " AND post_date >= '2013-05-01' AND post_date < '2013-05-8'";
    return $where;
}

add_filter( 'posts_where', 'filter_where' );
$slider_query = new WP_Query('posts_per_page=5&cat=3&orderby=highest_rated&order=desc');
remove_filter( 'posts_where', 'filter_where' )

assuming that you have this orderby=highest_rated covered with some plugin as you describe in the comment above.