Sometimes it takes what seems to be an inordinate time to progress through the scan. For a while the software is gathering information before it actually starts the scan. There could also be a problem with the hard drive disk-read which is delaying it or you may have a high propoertion of zipped files? Only a thought.
For the disk-read issue do a chkdsk /r. http://www.sevenforums.com/tutorials/433-disk-check.html
The software also sometimes benefits from running the Virtual Technician. http://mvt.mcafee.com/