Full scan stalls

Yesterday, May 25, 2015, I replaced Norton Internet Security with Norton Security at the end of the former's subscription.  The installation seemed to go without incident.

I tried yesterday to run a full system scan.  After about 90 minutes it just stops.  The scrolling of the file being scanned stops and the display just sits there.

I tried again today with the same results.  Further, I cannot stop the scan or close the scanning window.

NIS would scan the system in about 2 hours and, when selected, put the computer to sleep when complete.

I am not sure where to look to fix this.

The computer is an Acer Aspire running Microsoft Windows 8.1.

Thanks.