Norton Antivirus 2011 Scheduled Scan Problem

I recently installed NAV 2011 on a Toshiba Laptop running WinXP (SP3). It previously was running NAV 2010 WITHOUT ANY ISSUES. Now it has a problem with the regularly scheduled scan I set up to run every night starting at 5AM.

 

With NAV 2010 it used to take approx 45 minutes to complete the scan. Now it "appears" to either start late, or it's taking an inordinately long time to complete the same scan (approx 2.5 hours).

 

Sometimes it won't "complete" the scan unless I move the mouse pointer over the scan results display box. Then it "suddenly" completes the scan and posts the results.

 

Has anyone reported this problem? Has anyone else seen this problem? Does anyone know what could be causing this?