Why do you need to window service to run in background? Is there any specific reason to have the windows service for these requirements?
As per your detailed description what I understood is that when the employee completes a task and assigned to a user the user automatically get the message on his page. So when the employee is working and completed means they will click on some button that their work is completed. Then while clicking on the button, you just write the code for sending the mail to the tester. like the same way, when the tester is completed their work and click on the testing completes, send a mail to the reviewer and so on..Then where is the need to have a window service for this to send the mails?
All the requirements can be achieved through the simple send mail code.
Let us know if you need any more help or if you don't understand the exact scenario or if anything is missing in the requirements.
Hope it will be helpful to you.