To tell you the truth i aint much good with Apache.
PHP Warning: open(/tmp\sess_0a786498165e3ed3999c6cfa0f3d9289, O_RDWR) failed: No such file or directory (2) in d:\www\copyofheader\s.php on line 2
All i can think is that a file hasnt actually been created.
So thats that error associated with
session_start();
Your next error is somewhat associated with your first. Its undefined cos the Session was never created