Several options here.
1) Boot into Safe Mode. Keep pressing F8 during POST till screen appears with several choices. Choose Safe Mode (usually Option 3) Run ScanDisk from Safe Mode.
2) Download End It All from www.pcmag.com/article2/0,4149,1935,00.asp Use this to shut down all non-essential procceses.
3) Use CTRL+ALT+ Delete and 'End Task' everything except Explorer and Systray.
Any of these will let ScanDisk run okay. It keeps restarting because other procceses are writing to disk.