Norton Antivirus 2011 scan error

Hi everyone, I’m using Norton Antivirus 2011 on my Windows Vista OS and until now I have had no problems. It’s been a couple of weeks since I’ve last made a full scan so I tried to do it now, but it failed. The message that popped-out was this:
Full System Scan
Error During Scan
Results Summary
Total items scanned: 0
Total security risks detected: 0
Total security risks resolved: 0
Total security risks during attention: 0

What could be causing this problem?
Thanks in advance.
