Safe Mode

To go into safe mode you must restart your computer, and as the computer is booting up to that first screen,
 before it hits the windows screen, 
start hitting F8 until you get a menu that will give you some choices to go into normal mode, safe mode, safe mode with network support, etc. 
Use your arrow keys and key down to just safe mode. 
Then hit enter. 
Once into safe mode, your screen is going to look terrible, the reason for this is because when going into safe mode, none of the drivers are being loaded for any of your hardware.
You can then run Scandisk and Disk defrag with out anything running in the background.
Another thing that is good about Safe mode is, when your having problems getting your computer to start into windows, maybe because you loaded a program that was conflicting with something else, or some files have become corrupted, you can try to go into safe mode and uninstall what you just installed.
While in safe Mode you can usually do just about anything that you do when in windows, but its not a good idea, safe mode is just for fixing things, not for trying to run all your programs, or trying to get on the Internet..