Safe Mode
Use this emergency back door to Windows when you cannot start the computer normally. It starts a lite version of Windows that is not suitable for normal use but can be used to repair or restore the computer or backup data.
Last Known Good Configuration
Windows regularly makes copies of important system files and gives you the opportunity to restore the system if something goes wrong. You can either restore to the last time Windows started properly (Last Good Configuration) or choose an earlier date.
To start in Safe Mode or Last Good Configuration, start tapping the F8 key as soon as the computer begins to boot, do not hold the key down. You will get a screen like this:
Use your keyboard arrow keys to select the option you want and press enter.
System Restore
To use System Restore, go to Start>Programs>Accessories>System Tools>System Restore and choose to restore the computer. Select a date before the problem started and follow the instructions. You can do this from Safe Mode if the computer won't start normally.