how to install a system language on windows xp ?

hi all ,

i have a computer with windows xp

and i want to install a system language not a "input language" to my computer

the system came only with windows xp and i want to change it to the arabic language for the children to understand , when i tried it on the control panel and restarted the computer it didn't change , it still english

i have a windows xp OS CD with arabic but i cannot know how to install the arabic through it without re-installing it and losing data

wait for your help

thank you :)