How to stop blank entries

Hi All Firstly thanks to the people who have helped in the past with my questions... My issue: I have a registration script which allows new users of a site to become a member. All works very well BUT if a new user just hit enter on the blank form then a new account is created with all fields except the id field blank allowing anyone just to hit enter a gain access. I have search the MySql docs, Php.net docs, this and other NG's but i can't find the solution to this. And how can i carry forward the username to be shown on all pages after sucessful login. I thank you in advance Dave C To email me remove the NOSPAM bit of my return email address....