task scheduler won't run

task scheduler was working fine, but recently I have been
getting an error message upon boot/reboot that the tasks
could not run. not only are the tasks I set up not
running, but the automated tasks set up by windows update
and macafee for the virus scan & firewall to check for
updates are not working either.When I delete the tasks to
set up again, I get an error message: new task created
but may not run - object already exists - ox8009000f.
Any ideas? thanks