The Windows Registry is a collection of databases of configuration settings in Microsoft windows operating systems.
If you want to make changes system setting, first login your system with administrator account otherwise the changes would be apply only on user account. Every time when you attempt to make changes in the registry, you should firstly back up the registry. That’s why you need to know how back up the registry and how to restore it back.
You can save the backup copy to a location you specify, such as a folder on your hard disk or a removable storage device. If you make changes that you want to undo, you can import the backup copy. If you make changes that you want to undo, you can import the backup copy.
Find here step-by-step windows 7 support guide to easily backup windows 7 registry -
Step1: Go to Start->Run, type 'regedit'. Now windows 7 registry editor window will be open.
Step2: Now select left panel of window, go to 'Computer->Hit->File->Export'. You can save this windows registry anywhere you want to put it.
Now, you can make the changes you need to make to the windows registry. If after making the registry changes, you realize that, the changes did not give you the results you were looking for; you can simply restore the Windows 7 registry back to the point at which you backed it up.
Also find here how to remove a virus from your computer registry.