Just to add to what has been posted.

Scandisk and defrag
Programs running in the Background stop it.
Turn off screen saver.
Do Ctrl-Alt-Del and close all but explorer and try again.
Upon completion, suggest run defragment.

It can run in Safe Mode also.
Reboot and on startup hold the Control key [F8 if control does not work]
This will bring up the Boot menu
Arrow down [mouse does not work here] to 3. Safe Mode, hit enter
and you will boot to Safe Mode. [Yes it will look funny, very basic here]
[Screen saver should already be turned off] and run scandisk.
Upon completion you can run defragment also. It would be a good Idea as
you are already there.

Upon completion reboot to start you av and other programs

IN DOS
Scandisk can be run in dos also.
Defrag can not
Do the same as safe mode until you get to the startup menu.
Select 5. Command and hit enter.
and you will boot to the C:\ prompt.
Type in scandisk ,
looks like this
C:\scandisk
hit enter
when finish at the C:\ type win hit enter
or for a faster reboot do a Ctrl-Alt-Delete