>One thing to bear in mind, a task started with the task scheduler doesn't run the system logon script, so it typically doesn't get default mapped drives. The only reliable way I've found for a scheduled task to access files is via UNC. Don't know if this applies in your case.

This one runs on a local drive. Now, looking through event log, I see a security audit failure, seeing that there was something, launched by C:\windows\...\explorer.exe, that tried to run as Guest, and Guest was disabled. Tomorrow I'll try to give Guest some rights on the documents folder, just to see if that will change the outcome.