Scandisk (and also disk Defragmenter) require that nothing else be running while they are doing their thing. Some programs may not look like they are running, but they keep trying to start up, or update files (changing your hard drive) while Scandisk is trying to check it. You must shut down Every program that is running (or can start itself). If you can get into safe mode, then use it after shutting off any screensavers. I always uncheck the Show Clock box in Taskbar and Start Menu Properties. If you have problems using Safe mode, then proceed as follows:
Click START>SETTINGS>CONTROL PANEL>DISPLAY. In DISPLAY PROPERTIES, click the SCREENSAVER tab, and select NONE. Click APPLY and then OK.
Next, press the Ctrl, Alt, and Delete keys (all 3) at the same time. Ctrl+Alt+Del will bring up a Close Program window. One of the programs listed will be selected (made blue). If the program is NOT Explorer, press the END TASK button at the bottom. (If the program selected is Explorer, click down to the next one before using END TASK.) The window will vanish. Press Ctrl+Alt+Del again and close the next one. Close every program except Explorer. When Explorer is the only program left, use Cancel to leave the Close program window.
Finally, right click on an empty area of the taskbar. A pop-up window will appear. Select properties at the bottom. A "Taskbar and Start Menu properties" window will appear. Uncheck 'Show Clock', then click OK.
Now you can start Scandisk or (Disk Defragmenter). Do not touch your mouse or keyboard while it is running -- it will take a while. ( Go make a cup of tea or something) Restart your system after it is done. Good luck --911