Full system scan gets stuck

I am using Norton AV 2011 and I had a full system scan scheduled for today and the scan started , I came back to my computer a half hr.  later and there showed 127,xxx files scanned, watched the scan for 5 minutes and the number did not changed , touched the mouse and the scan numbers started going again and the scan finished . Have had AV installed for a few week and this has not happened before . I started another full scan manually and this time it went 5,xxx files scanned and the bar still showed scanning but the number did not change for 10 minutes and moved the mouse and file number scaned started going again. Any idea what going on ?

 

Windows xp, service pack 3 

Zone alarm pro firewall