If a scan or defrag hangs, it could be due to some bad sectors on the drive(s).
To check your hard disk, click on the Windows Start button. In the search box type CMD. Right click on cmd.exe and click on run as administrator. In the command prompt type "chkdsk c: /r" without the quotes. A message will come up asking if you want to check the disk on the next restart. Answer 'Y'. Reboot and the check disk will run before Windows starts up. This check will check the file system as well as the physical disk surface. This scan can take a long time, depending on the size of your hard drive.
I would suggest running this on all your drives that you had checked for NU to defrag.