When cron.php does... nothing !

I am using Drupal 5.1 on Linux. When i execute cron.php logged as administrator, my navigator displays a blank page... and when i consult ?q=admin/logs/status, it displays:

Cron maintenance tasks Never run
Cron has not run. It appears cron jobs have not been setup on your system. Please check the help pages for configuring cron jobs. You can run cron manually.

Can somebody help me?

Thanks for your answers.