The path to watchdog error logs (D6):

http://www.example.com/admin/reports/dblog

To clear the watch dog error log, use these MySQL commands:

(Change path to MySQL specific to your server)

"C:\Program Files (x86)\acquia-drupal\mysql\bin\mysql" <Database Name> -h 127.0.0.1 -u root -e "DELETE FROM watchdog;"
"C:\Program Files (x86)\acquia-drupal\mysql\bin\mysql" <Database Name> -h 127.0.0.1 -u root -e "ALTER TABLE watchdog AUTO_INCREMENT = 0;"

It is critical that auto increment be reset if you truncate the watchdog table; not  doing so may cause errors when viewing the log from within Drupal.