Scannow is not meant to run from inside Windows.
You type in "sfc /scannow" at the cmd prompt... Start Menu, Run, type in "cmd" (without the quotes), then hit enter.
Now type "sfc /scannow" at the cmd prompt (without the quotes), hit enter. Exit out of the cmd window (type "Exit" (without the quotes) hit enter. Then re-boot the computer. Scannow will run during the Windows boot process.
Hope this helps.