Moved to VirusScan to be with the other posters reporting this problem.
Have you got use minimal resourses selected in the scabn settings.
Is this 2010 or 2011 version?
2011 has an about link top right of the GUI front page.
If it is 2011 you can in scheduled scan settings exclude folders and files that you know are clen ie large foders of jpgs and compressed files. The 2011 version also scan 2x speed as it scans using multi scanning threads.