If you activate the Tamil "IME" (turn on its keyboard), then Windows
will offer you several Tamil fonts. (Tamil is one of the languages for
which a number of alphabets are included. Thai is another.)
Go to Start > Control Panel > Regional and Language Options, and then
follow the tabs and instructions for adding a "Keyboard" (not for
adding a language) to Windows. It works a bit differently in Windows
XP, Vista, and 7.
With Windows XP, you may need to have your Office distribution disk(s)
on hand.
When you've added new language keyboards, you'll have a new icon at
the lower right of your screen. Initially it will show "EN." Click on
that and it pops up with a list of the installed keyboards.
With the cursor in the Word document where you want to type Tamil,
choose the Tamil keyboard.