It might be time to check your hard drive.
Push the Windows button on your keyboard and type Command Prompt in the search box, right click Command Prompt, Run As Administrator. Now type chkdsk /r . Note the space between the k and the /.
Next time your system starts Check Disk will run and check your disk for bad sectors. This could take quite a while (hours) so best to do it when you know you won't be using your computer.
When that has completed try running NU16 again.