So, when you remove a computer from a domain and a user from the domain has a scheduled task on your computer, you can't delete the task anymore…at least not through the Task Scheduler interface.
"Error message: The specified account name is not valid," is the error I was getting.
The answer lives in the registry. Find your task among the unique ID's here…
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Tasks
Then delete the actual task here…
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Tree
That'll do ya.