In windows all the important details about the installed software and details about the windows are registered in a small database call Registry. The main purpose of the Windows registry is to store the configuration of the different software programs installed on the computer, the hardware installed or attached to the computer, and any other special settings the user has implemented. The Windows Registry is a crucial part of your PC's operation system.
A computer's registry is an efficient way of monitoring the different software and hardware components which is use in the computer. For example, all the software programs are organized based on user profiles.
The computer's registry can become corrupted by spyware and viruses. By simply creating a .vb script we can easily edit the windows registry entries. In addition, with continually installing and removing software, not all programs will remove themselves cleanly and completely from the registry. Therefore in over time, the Windows Registry can begin to contain information that's no longer valid. Maybe you uninstalled an application without using the Add or Remove Programs function in the Control Panel, or perhaps an object or file in the registry got moved. Eventually this orphaned or misplaced information accumulates and begins to clog your registry, potentially slowing down your PC and causing error messages and system crashes. Mainly you might notice that your PC's start-up process is slower than it used to be.

So cleaning the registry is an essential step to speedup your PC. So how can you clean your register?
Method 1 : To manually clean your registry using the Windows Registry Editor, first create a backup of the registry, and then remove unnecessary files :
   *   On the desktop, click the Start button on the left bottom corner    *   Click Run    *   A new window to "run" commands    *   Type regedit    *   Click OK    *   At the top of the window click File    *   Go down to Export    *   Type a name for the backup file, and click Save
After creating a backup of your registry, you can go to the left side of the registry editor window and view how your computer is configured.
Click on the plus sign to expand a section of the registry editor. A "well-used" area for anyone manually deleting registry files is "HKEY_LOCAL_MACHINE." This area is used to make registry changes to hardware and software.
[ You might think this method is something sound like dangerous and not effective. Yesh.. Because you do not know about the entries which are necessary and unnecessary data in the registry. So I do not suguest you to use this methode. :) ]
Method 2 : Using some registry cleaning software. This is a good method to clean your registry entries. I always use the CCLEANER software to clean my windows registry and temporary data in the PC. CCleaner is a freeware system optimization, privacy and cleaning tool. It removes unused files from your system - allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history. Additionally it contains a fully featured registry cleaner. But the best part is that it's fast (normally taking less than a second to run) and contains NO Spyware or Adware! :)

CCCleaner is cleans the following unused and unnecessary items from your computer. Internet Explorer Temporary files, history, cookies, Autocomplete form history, index.dat. Firefox Temporary files, history, cookies, download history, form history. Google Chrome Temporary files, history, cookies, download history, form history. Opera Temporary files, history, cookies. Safari Temporary files, history, cookies, form history. Windows Recycle Bin, Recent Documents, Temporary files and Log files. Registry cleaner Advanced features to remove unused and old entries, including File Extensions, ActiveX Controls, ClassIDs, ProgIDs, Uninstallers, Shared DLLs, Fonts, Help Files, Application Paths, Icons, Invalid Shortcuts and more... also comes with a comprehensive backup feature. Third-party applications Removes temp files and recent file lists (MRUs) from many apps including Media Player, eMule, Kazaa, Google Toolbar, Netscape, Microsoft Office, Nero, Adobe Acrobat, WinRAR, WinAce, WinZip and many more... Click Here to Download CCCleaner software.. enjoy :)