Where's my scheduled Backup?

I have my Dell 4400 set to do an incremental backup every week. I am backing up to a 200G Western Digital external drive thru a USB PORT. I am using M/S xp/Home backup software that shipped with the computer. The schedule Backup Utility (scheduled jobs) calendar looks correct. Every Friday has a little icon inside of the Friday date indicating it is ready to go but it never fires off on its own. I can do a manual backup no problem but I don't want to have to think about it. I just want it to happen. Suggestions anyone? Thanks dmeans43