Before resetting your Registry, read about Understanding and Editing the Registry.
The Registry is the place where all your hardware and software information is kept in Windows. Windows still uses win.ini and system.ini for backward compatibility, but there is a great deal more information kept in the Registry.

Below is a step by step guide to resetting the Registry. The complete Registry contains 2 main files and 3 backup files. The main files are: SYSTEM.DAT (the Registry system file) and USER.DAT (the Registry user file). The backups are SYSTEM.DA0 (as in zero), SYSTEM.1ST and USER.DA0 (as in zero). SYSTEM.DAT, SYSTEM.DA0, USER.DAT and USER.DA0 are all kept in the Windows directory, while SYSTEM.1ST is located in the root directory.

The Registry system files take care of hardware and software. These files keep up to date on what's on your computer.

The Registry user files take care of user specific settings. What your wallpaper is, whether profiles are used and who gets what profile. Normally, the Registry user files are not changed when resetting the Registry.

There are two ways to reset the Registry; a basic reset and an advanced reset. The basic reset is listed first, and should be tried first.

WARNING: Resetting the Registry will force you to reinstall some or most of your software before you can use it again. This removes the references of your software from the Registry, but does not remove the shortcuts on your desktop. Even though it looks like the software is still on your computer, you may need to reinstall before using it. Please make backups of your computer before trying this.

BASIC RESET

1. Rename the main Registry system files and copy the backup registry file. There are two main Registry system files: System.dat and system.da0. These two files reside in the {Windows} directory and are hidden, system files. To get to these files, you need to start the computer and go to a DOS prompt. From there, change to the {Windows} directory. The files will not be seen, because they are hidden. These files need to have their attributes changed. To do this, type attrib –r –h –s –a system.*. This will reset all attributes to all files called system in the {Windows} directory. Once these file attributes are changed, they can be renamed. To rename the proper files, type ren system.dat system.xxx. This will rename the main Registry file (system.dat) to a backup name. Next, rename the backup system file by typing ren system.da0 system.xx0. System.da0 is the backup file for system.dat. If you have a registry problem, and you catch it before you reboot your system, you can replace system.dat with system.da0 and that should fix the problem. However, if you reboot your computer, system.da0 will get updated and, therefore, become corrupt as well.
2. There is a third Registry file that is created when Windows is first installed on the computer. If Windows came pre-installed on your system, you will still have this file. This file is in the root directory, so you need to get to that by typing cd\. This will put you right to the root directory. From there, you need to reset the attributes to the third Registry file by typing attrib –r –h –s –a system.1st. Yes, that is the number 1. This file  is created when Windows first installs. To use it, type copy system.1st c:\{windows}\system.dat. By doing this, we copy the contents of system.1st into a newly created system.dat file. This new system.dat file effectively resets the main portion of the Registry for you.

   Reboot the computer and try it. If this doesn’t help, go to step 2.

   ADVANCED RESET

3. Rename all Registry system files and reinstall Windows. If the above doesn't work, there may be a Registry problem that doing a basic reset won't help. If that happens, use the steps in number 1 to rename all Registry system files and then reinstall Windows from scratch. Remember that you will need to either access to the network from a DOS prompt or you may need DOS access to your CD-ROM drive.