try this one:
To set up Arabic under Windows XP, select Start | Settings | Control Panel | Regional and Language Options. Click the Languages tab at the top; check the box which allows you to "Install files for complex script and right-to-left languages (including Thai)”, Apply, then click on Details... under Text services and input languages. When the window appears, click Add... next to Installed services and select Arabic (Egypt) and Arabic (101) as your Keyboard layout/IME. Click OK, Apply, Ok and you're done.
To set up Arabic under Windows Vista, select Start (or windows logo) | Settings | Control Panel | Regional and Language Options. Choose "Keyboards and Languages" at the top, click on the button "Change Keyboards", then "Add", scroll to where Arabic is, you might want to choose "Arabic (Egypt)" for example, make sure you click on the + icon before the word "Arabic (Egypt)”, once you do, you will see "keyboard" click on the + icon placed before it. Check the box for “Arabic (101)”. Click "Ok", then "Apply" and finally "Ok", and that's it.