if you're on windows xp or windows vista, go to control panel, find regional settings, from there if you take some time and play around you'll find a place to add additional input languages for the keyboard. when you're done you probably need to restart. to switch between arabic and english you press left alt-shift

do u mean, u want to write arabic using ur english keyboard???i really dont know how but i have other option, u can use the arabic keyboard offered by these sites:www.islamonline.net or www.islamweb.net. u can write the text u wish then copy and paste it, tedious ?? yes but it can help.