What is a runtime error? It is an indication that your system has become unstable due to broken registry links or some other problem. When a program stops in its tracks, you will see this error. The program could stop due to a virus, insufficient memory or conflict with another program. Running several programs at the same time can cause runtime errors. Most technical people understand what this error is and how to get it fixed, so it is best to consult a professional whenever you encounter it, especially the runtime error 21.
One of the first things that you need to do when fixing this error is to set your computer in "Safe Mode". When your system is in operating in Safe Mode, Windows is able to go into your PC without interference from third party settings and applications. By doing this, you will be able to isolate the problem and check if the error is due to Windows itself or if it's happening because of some bit of software installed on your PC. When setting your PC into Safe mode you need to first restart your computer. Then before Windows can load itself on your system, press F8 repeatedly. Your screen will then display the Boot Options menu, from which you can select "Safe Mode with Networking." Following this, the "pure" version of Windows will then be loaded on to your computer, which will allow you to check for errors with any of its options or settings. Run your computer in this mode for around 30 minutes.
The last step to fixing this error is to then clean out your PC with a "registry cleaner". These software tools basically scan through a part of your system called the registry - which is a large database which stores vital settings & information inside. This database is where the likes of your desktop icons, latest emails and even your Internet Bookmarks are kept, allowing your PC to store a huge number of vital settings which it requires to run each day. It's often the case that the registry will cause Runtime errors on your PC, which is why it's recommended that you clean out your system by using a registry cleaner program.