You either don't have a /tmp directory, or your web server doesn't have permission to write to it.
Since your error is showing 2 different styles of directory navigation / for unix and \ for dos I'm assuming that you are running on a Windows machine with a standard setup.
What you need to do is look in your php.ini file, if you don't know where it is, you can use a phpinfo() page to tell you. Look in this file for the line:
session.save_path = /tmp
Then just change this line to be something that your system recognizes as a temporary directory.