From here: https://superuser.com/questions/579900/why-cant-excel-open-a-file-when-run-from-task-scheduler

1. Open Component Services (Start -> Run, type in dcomcnfg)
2. Drill down to Component Services -> Computers -> My Computer and click on DCOM Config
3. Right-click on Microsoft Excel Application and choose Properties
4. In the Identity tab select This User and enter the ID and password of an interactive user account (domain or local) and click Ok
Keeping it as the interactive user or the launching doesn't work with the task scheduler unfortunately, even when setting the task up to run under an account that has admin access to the machine.