Lets eliminate the silly easy stuff first.
Is your system clock correct? If it is set to 2001 it might be a while before your Friday 17 November 2006 VSS runs.
Have you checked if the Scheduler Service is running and functioning? Do any other scheduled tasks work (batch files of any sort)?
What about permissions? Is the account used for successful manual backups used to configure the scheduled jobs?
Could any installs still be incomplete (need reboot) or hung in the system? Is COM+ clean and uncorrupt?
Do you have all the VSS hotfixes?