Windows Server 2003 SP1 (or higher): if your Scheduled Task is giving you back a result code of 0x80 (instead of 0x0 what would signal that everything's okay), then there's a plethora of possible causes. If you google a bit, you'll find many solutions ranging from exchanging a DLL up to notices that 0x80 wouldn't mean anything at all and that you don't have to care about.

Another possible reason for a 0x80 result code is lack of resources. When you encounter the 0x80 problem, read this fine discussion (I've seldom seen a problem description with so much detail like there) and have a look at the Task Manager of your server. You might be surprised like I was this morning. I erased all crashed tasks and presto! problem solved.