Most commonly used English fonts, such as, Arial, Times New Roman, give acceptable results when used for Arabic script. Some installations of Windows also include fonts such as Arabic Typesetting and Traditional Arabic, which are more suitable for Arabic script and provide quite good results. See the bottom of this post for a sample Arabic script in various fonts.
My favourite one, which I use extensively for the Arabic class is called Scheherazade. You can download it from the following site.
Make sure you read the license terms before downloading and installing the fonts.
Once downloaded, extract the contents of the zip file into some folder.
Open Control Panel and then open Fonts.
Then right click on some empty space in the right pane and select Install new font…. Alternately, press Alt key to view the main menu, then select File -> Install new font… option.
Then navigate to the folder where you extracted the new font files. The new font should be listed in the List of fonts. Click Install.
Please note that if you share your MS Word documents or Powerpoint presentations containing the new font, e.g. Scheherazade, with someone who does not have the same font installed, they will see the text in some default font. Most of the time, it looks quite nasty. While asking the other user to install the appropriate font is one solution, you can also save your documents as PDF and share those. In that case, the other user does not need to install the font.
Here is a sample of the various fonts, mentioned above. Font size of 28 was used for the Arabic script.
Hope it was useful information. If you know of other good Arabic fonts, please share.