Naveed, I do this on all my forms so that when a user checks a box it shows their name in a field to show who did the task, because some of the forms have different users doing different tasks on the same form section before submission.  They just save a draft and reassign it to the next person.  The way I have the current user displayed in the user name field is via a user table I setup in SQL.  Each row only has two things, the user's display name, such as CLS\Rick.Nagy, and the user's name (Rick Nagy).  Simply put a field on your form with something like 'Currently Logged in User' for the name.  My two values in the table are 'user_name' for the display name and 'empSiggy' for the employee's actual name.  Here is the lookup rule:

This will always fill the box with the current user, rather than the user who was working on it before.  If you want to keep track of all users, have a name field for each task, and add a javascript function to put the current user name in the field when a box is checked, for example.

Rick