Problems using WP_PLUGIN_DIR in plugins

I require getting the absolute path of a file in a specific plugin, and being able to refer to this path in different plugins, and possibly a page template here and there.

On the Codex I came across WP_PLUGIN_DIR // full path, no trailing slash which would fit my needs but the Codex recommends not using this constant in plugins..

I echo’d the constant to see just what it contains and got the following “C:\xampp\htdocs\twitter/wp-content/plugins”, which brought up a second issue, why is there both backward and forward slashes?