Without understanding the whole context, I am of the suspicion that you have logged in with a users [desktopmachinename/aspnet] being a username on the machine. if this user doesn't have rights to make any changes (like admin rights), the machine will challenge you for the login details.
Have you tried to switch user logins.
option 1 : Press the win Key + L. This will take you to the login screen. you can try to re loging
option 2 : Right click on the task bar (not on any icon) click on Task Manager. then select Users tab which will list all the users on the system. You will have a sign out or switch user button. click on switch users and follow on screen procedures.
option 3 : Press on Ctrl-Alt-Del then click on switch user and follow on screen
I hope that helps