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