My Internet Security had been running perfectly at the scheduled time until I renewed my subscription by buying a new copy and entering the new key. I probably also ran Live Update about the same time. I tried rescheduling, nothing worked.
Today I looked at the Windows (XP SP3) scheduler and saw 2 entries for Internet Security and both said the task was unable to run.
In checking the path in the scheduler both entries had 16.5.0.135 as the version sub-directory. I then checked the path with Windows Exporer and found the directory name was 16.7.2.11 which I found matched the version of my Internet Security.
I deleted both entries in Windows Scheduler and then resceduled the scan in Internet Security. Now everything looks good in Windows Scheduler. I'll wait for the scheduled time to see if it works, but I have a good feeling.