Maren Speller is an online tool as well as a downloadable desktop application from Microsoft Research.

No more Arabic spelling mistakes! Most of the Arabic content has a high rate of spelling mistakes. Around 95% of these mistakes are common Arabic mistakes like confusion between the different types of “Hamza” and the “Haa” and “Taa marboota”. Other mistakes include collated words, wrong character insertion, swapping, deletion, repetition and replacement. This utility is a full-featured Arabic spell-checking tool that works directly within Windows 8. It automatically corrects all these mistakes with high accuracy.
How to enable/disable Maren Speller after you have installed the downloaded desktop application.

By default, the setup offers to replace the Windows Arabic speller with Maren Speller. If it is needed to get Windows speller back as the
active speller, here are the steps to enable or disable Maren Speller as the active Windows speller:

Press Windows logo key +C to open the charms bar
Click Settings and then Control Panel
Open Language, a window similar to the below will appear
For Arabic language, click Options
Under Text services section, you could choose which speller to be active in Windows.

Online Tool | Download application.