If it is a laptop unplug all the external device, power cord and battery then plug in the battery & power cord again restart the system
If a desktop unplug external devices start system then start tapping F8 use safe ode to boot to the desktop and then try to perform system restore is it fails go back to safe mode click start then run and in run box type msconfig then hit ok. When u get system configuration utility go to the tab start up and click disable all. Hit apply then close and then restart the system.