Windows® 2000 does start
Log on to Windows® as Administrator.
To restore the registry to one of it's previous states, you can use either the Start Menu -> Registry First Aid group -> Registry restore shortcut or run the Backup utility from the Programs -> Accessories ->System Tools menu.
In the Backup utility select the Restore tab, choose the backup media, and then select the System State check box. Keep in mind that Backup will not only restore the registry, but it will also restore all of the system state data.
Note that after the System State restoration process is completed, you need to shut down and restart the system. You won't see the results of the restoration immediately.
Wizard Tip: When you start Windows® 2000, you can instantly restore the most recent copy of the registry. To do so, restart your system and when you see the "Please Select the Operating System to Start" message, press [F8] and then use arrow keys to select the "Last Known Good Configuration" option and press [Enter]. You can also use this quick restore technique in Windows® NT.
Windows® 2000 does not start
If Windows® does not start, restart Windows® by using the "Last Known Good Configuration feature": start the computer and then press the F8 key when Windows® begins to start. The Windows® Advanced Options menu appears.
Use the ARROW keys to select "Last Known Good Configuration" (your most recent settings that worked), and then press ENTER. Windows® will restore the computer to the most recent restore point.
Warning! Make sure that your backups are current. If you restore an old backup it could be fatal if you've made major changes to the operating system or hardware since the backups were created. For example, if you've installed Internet Explorer 6 and restore your registry from a backup predating that, expect that your computer won't boot properly.