To enable Thai language fonts to work with Windows XP SP2, first select 'Control Panel' from Start Menu.
Then select 'Regional and Language settings'. Then select 'Languages' Tab.
Then check the first or both check boxes* in Supplemental language support.
You then need to select apply button and you will be prompted to re-start you computer.
Re-start the computer to save these Windows settings changes.

* "Install files for complex script and right-to-left languages (including Thai)."
   
    If this does not help check the second box:   "Install files for East Asian languages".


If you still can't see the Thai script in the right window, please make this check;

Control Panel >>> Regional and Language Options >>> Languages Tab >>>> click on the "Details" button and a new window will open called "Text services and input languages". In the box you will see "English" and also "Thai Kedmanee" or another "Thai". If not, Look for "Install services" and click on "Add" button. A small window will open with two drop-menus where you can sellect a  language to add. On the lower drop menu select "Thai Kedmanee".