loginusername / password? win service

Question

I m trying to learn win services... I have created a very simple winservice in C# (my first one infact), i have made a setup file for it and when i am installing it asks me for set service login username and password... i have tried givin it various usernames and passwords but if my pc doesnt' have a username and password, why am i being asked this question... what is this username and password about..

thanx for ur help..