Full System Scan Didn't Scan at Scheduled Time

Today the Full System Scan did not do a scan. I have it scheduled to do do a full system scan at 8AM weekly every Saturday. I did it manually, and it worked fine. Any ideas why it didn't come on at scheduled time. I checked settings and they were ok. I would like to know if Only at Idle Time should be checked as this is unchecked, and at one time it might have have been checked. I never use my computer when a Full System Scan is scheduled.

Last week I had another problem---my computer shut down on its own during the Full System Scam. I believe this happened another time during the Full System \Scan. I searched for this problem in the Norton search and evidently someone else had this problem not long ago.

Windows Vista