Watchdog "404 page not found" entries from spambot guessing urls
Block them using these modules, study these and related modules for perfect working solution to your need :

http://drupal.org/project/httpbl

http://drupal.org/project/badbehavior

http://drupal.org/project/antispam (Apart from blocking comment spam, disables bot showing 403 error)

http://drupal.org/project/botcha

Install http://configserver.com/cp/csf.html and use Real Time Block Lists (RBLs)

And finally a free-service like http://www.cloudflare.com/features-security will get things working.