I don't know what OS you are running but here are a few articals from the MS site.
WinXP
Linky 1
LInky 2
Win98
Linky 1
Linky 2
<H2>More Information</H2>Under normal circumstances, Windows is capable of detecting and recovering from registry errors automatically. If Windows is incapable of this, a previous copy of the registry can be restored manually. Windows makes and stores a backup of the registry when you start your computer successfully each day. By default, five previous copies or the registry are stored. To restore one of these previous copies:
<OL class=dec>
<LI>Start your computer, press and hold CTRL, and then choose Safe Mode Command Prompt Only from the Windows 98 Startup menu. If you are running Windows Me, start your computer with the startup disk.
<LI>At the MS-DOS prompt, type <KBD>cd\windows\command</KBD>, and then press ENTER. Where <VAR></VAR>is the name of the folder in which Windows is installed.
<LI>At the C: prompt, type <KBD>scanreg /restore</KBD>, and then press ENTER.
<LI>Select the previous registry you want to restore, and then press ENTER. NOTE: A properly working registry has the word "Started" next to the date.
<LI>When you receive notification that you restored a properly working registry, press ENTER to restart your computer.[/list]