Safe mode is different from the usual that when you start the system is loaded only the necessary programs and drivers. As a rule, in safe mode you can boot even with a fairly serious crash.
To start the computer in safe mode, press at startup F8. It is best to press the button several times with a frequency of about once per second - this will allow you not to miss the moment. A window will appear with options for starting the operating system. First try the option "last known good configuration" in many cases, this option allows you to correct all the issues.
If the computer refuses to boot in this option, you need to try safe mode. String of its choice are at the top of the window. You will have three options: "Safe mode" "Safe mode with command prompt" and "Safe mode with networking". Select one: in this case, you get quite familiar with the Windows interface. Only the screen will be black at the corners you will see "Safe mode".
If you select "Safe mode with command prompt" instead of Windows Explorer will run the console (command prompt). This mode can also be useful, but with the command line, you must be able to work. If you opened the console for list of commands type HELP and press Enter.
How can you use safe mode? If you can only boot it, try to run the rollback to an earlier state of the computer: "start" - "All programs" - "Accessories" - "system tools" - "system Restore". If the rollback succeeds, after a reboot you will get a restored system. Recovery is possible if the computer was previously created restore points. When working with the console launch recovery you can invoke the command: %systemroot%system32 estore strui.exe.
Safe mode is useful if the computer has several people, each created their accounts, and one of the employees forgot the password to login. In safe mode you will enter as an administrator and will be able to easily change your forgotten password. To do this, open "control Panel", find the section "user Accounts", select the account and change the password.