im the only user on my computer and i don't need a password, so why is it so difficult to set up a task????

all i want is a program that will run in the middle of the night and change my desktop wallpaper each day. When i had my program to the task scheduler and try to run it, it tells me my password is not right (apparently i have to set a password if i want to run tasks). i even made a seperate username that had a password so i could run the task, but then i have to go through that useless log on screen each time i boot up and also the program is run as the other user, so it change's their wallpaper, not mine!!

this is so freakin annoying, anybody know what to do (besides having my program always running in the background and checking what time it is)??