On submit code

To execute code when a webform is viewed you can create a module that uses the node api hook and executes code when a node (with type webform) is beeing viewed.
(for more info see http://drupaldocs.org/api/head/function/hook_nodeapi)

Executing code on submit could also be handled using the node api but I guess a better (read easier for the administrator) would be to create a new webform specific hook and call it just before submitted data are stored in the database.