Here are the step how to enable UNICODE 

Unicode by definition provides a unique number for every character  irrespective of the language, operating system or program.  (Although Tamil and most Indic languages does not actually provides unique number for every character but only chracters are displyed by combining the Unicode charters.)  Unlike ASCII (American Standard Code for Information Interchange) which is merely for English and most of the European Languages UNICODE covers almost all languages spoken in the Planet. Windows Vista have built in UNICODE support Including Sinhala. 

To enable UNICODE support in Windows XP 

Logon to a system as administrator or with administrative previlages.

* Start -> Run ->intl.cpl 

Alternatively you may go to Control Panel by choosing Start -> (All)  Programs -> Control panel. Choose Regional and Language Options

Go to the Language Options and put a tick before Complex Script (Although Tamil is Technically not a complex script but this is how it is classified in Windows XP)


Ok it will prompt you to insert Windows XP CD just insert any Windows XP CD (need not be an English version any laguage version like german or even evalution copy of windows xp will do the trick) .

That's it you need to start the computer now computer is UNICODE enabled.