Arabic Keyboard 5000 is easy and fast to use.
Note: the official Clavier Arabe CO website is down and it looks like that the software hasn't been updated for several years. The version linked to this page is one of the latest versions of the product. It seems highly unlikely to have any newer versions.
Arabic Keyboard 5000 is an application that gives you access to an onscreen keyboard with the Arabic alphabet even when using a computer with a keyboard in a different language.
It can also be used as a search engine and a translation tool between Latin and Arabic characters. In some cases, you can even browse the web from directly within the application's interface.
Arabic Keyboard 5000 is a free tool.