There are two ways:
1- There is a program by which the english keyboard is mapped to arabic letters, you can write inside the program and then copy it to send emails. But this is not the approporiate method and don't recommend it unless you are in a hurry.
2- Activate the program and follow the following steps to install the system files
a- Right click on the program icon in task list.
b- Choose Arabic Enable or click Ctrl+Shift
c- You will see a window where you can write arabic in. Click on Install System Files button.
d- You will see a new window showing the installation process. Click on Install All Arabic Keyboard Package.notice, if you are in the West, don't make the arabic language your computer default language. (keep this option unclicked)
e- Reboot the PC
f- You will see a small En/Ar icon in the task list, click on it to change the language from/to arabic. you can also click left Ctrl+ left Shift to change to arabic. To change the writing direction click on Left Ctrl+Right Shift.