On Windows XP:
Click on "Start->Control Panel->(Date, Time, Language and Regional Options)->Regional and Language Options" Click on 'Languages' tab (next to 'Regional Options') at the top In 'supplemental language support', tick the "Install files for Complex Script and right to left languages (including Thai)" and install the required files.
Windows XP CD may be required for the above process.
On Windows 2000
Click on 'Start->Settings->Control Panel->Regional Options' Click on 'General' tab You will see a list of languages under 'Language Settings for the system'. Scroll down the list to tick 'Indic'.
Windows 2000 CD may be required for the above process.
Windows XP and Windows 2000 System files contain a Tamil Unicode font called "latha" that can be used to view Tamil Unicode texts.