Heres one way of adding a language if you haven't already:
1. click "Start"
2. go to "Control Panel"
3. open "Regional and Language Options"
4. click on the "Languages" tab at the top
5. click the "Details" button
6. click the "Add" button in the center and add whatever you want and click ok.
7. in the bottom right corner of your task bar you should see a new button there (it will probably say EN or AR). click on that to switch between languages that you've added.
Using microsoft word, try pressing the ctrl+shift combination I don't remember either left or right, but you have to have the arabic language enabled in your windows.
Think the default is left alt + shift, or at least for me it is.