Windows 7 Task Scheduler is giving me an error saying "The specified account name is not valid"?

The issue was after changing my PC name, tasks I had created in Task Manager still had the User in Security Options set to the old PC name. The way to fix this was for every task:

Task Manager > Right-click Task > Properties > Change User or Group... > Enter the object name to select.

For the box "Enter the object name to select" I had to type my account username, not my new PC name. For example, "Alex".

One final note, the "Author" section for the Task will still show the old PC name, but that caused no issues for me.

I know this is old, but none of the solutions above worked for me. I ended up discovering that you have to enter the user name as ComputerName\Username in the text box (along with the password, of course) even if it is a local account. Once I did this, it worked perfectly.