We'd like to know the recent changes you've made to your system which is the most suspected reasons of recent failure. Also, can you please confirm that if you've Windows isntallation disc available at you. If so, consider booting your system with it and go with 'Repair Your Computer' instead of full installation.
Follow the same steps 2 or 3 times to troubleshoot the problem you've stated. Don't hesitate to post back if the problem still persists!!