To add some additional information, it would appear that scheduled tasks are stored at the following location in the registry:
HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule
Changing the credentials (username and password) used to run a particular task results in changes to the following values:
HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\CredWom\x-n-n-nn-nnnnnnnnnn-nnnnnnnnnn-nnnnnnnnn-nnnnn\Index
HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Tasks\{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}\Hash
HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Tasks\{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}\Triggers
However, changing solely the password of an existing user account that is used to run a scheduled task results in no apparent changes in any values below the "Schedule" key. As a matter of fact, I see no associated changes anywhere in HKLM.
Using Process Monitor, I can see mmc.exe accessing the "schedule" key when a task is created or modified. I can also see svchost.exe accessing the same portions of the registry, which is probably the operating system updating the Task Scheduler service (netsvcs), itself. The lsass.exe writes to the SAM file when a task password is updated. I do not know if it is simply verifying the password entered in the task at that point or if it is somehow storing (or linking) the task credentials in the SAM.
I am still not certain where or how the password is actually stored for scheduled tasks but it does not appear to be stored in the registry--at least not in HKLM--which is a good thing.
