I am scanning an external 50 GB hard drive, of which only 15GB is used, and the scan is still running after 36 hours. Is this normal or is something wrong?

My computer is a Pentium Core 2 duo, with 2GB RAM, running Windows XP pro SP3. The drive being scanned was taken from an Acer laptop believed to be infected. Having been removed from the laptop it is connected to my computer using the external SATA port. I am using Spybot version 1.6.0.30. I started Spybot from the context menu in Windows Explorer by right clicking on the drive icon and selecting "scan using spybot".

My task manager shows Spybot file scanner running. My CPU's are running at 40-60%, RAM usage 50% (1GB). There is a little window visible on the desktop listing lots of files, some of which are "waiting" and some say "nothing found". There is an empty progress bar at the base of the window and just above that a message saying scanning <filename>. The filename changes from time to time, giving the impression of activity, albeit at a very slow pace.