Drupal 6 webform: send email if a checkbox is checked?
You can use the rules module to send an email if the checkbox was checked


Install drupal [Rules module][1]
Install drupal [Webform Rules][2]
Create a rule
On condition section you need to custom PHP to check if the checkbox was checked 
Make sure that you can access the field in data selector
Chose the operation equal
For the "Data Value" field, you can input values in two modes, make sure you are using "direct input mode". You should then be able to check the checkbox that represents TRUE for that field. Translated, what you are saying is, "Is this field equal to TRUE?" You can also negate it by leaving it unchecked or using the "negate" option below.