Microsoft

Apparently, the task scheduler in all Windows NT-derived versions up to (and including) Windows Server 2003 cannot run jobs under the "NT Authority\Network Service" security principal; "NT Authority\System" is the only so-called well-know security principal that can be used with it. Support for running tasks as "Network Service" or "Local Service" was only added in Task Scheduler 2.0, which is the version that ships with Vista and will be also included in Windows Server 2008.

Side note: I wonder why the link above, being a command reference for an administrative tool, is on the MSDN and not on the Technet site.

Practical implication is that without being able to use the "Network Service" principal, a distinct network account (domain account or server account) is required for any job that needs to access any network resource. This also means keeping track of another account and password — and taking care of controlling and auditing it's access; whereas the "Network Service" principal cannot normally be used for interactive sessions.

Annoyingly, the task scheduler UI also does not tell you that you can't if you try, but rather gives the un-helpful message "Access is denied. You do not have permission to perform the requested operation".
The same message is also shown when, for a regular account, the password entered does not match the actual account password. (This happened to me because the keyboard layout on a remote desktop session turned out to be not what I thought it was)
Anyone know if the error reporting has improved anything in the new task scheduler?