Then you have to install "Unicode font" in your computer. You can Download "Sinhala Unicode Kit" referring this link .
http://www.siyabas.lk/files/SinhalaTamilKit_XP.exe
Download it and install.
Sinhala Gini Nariya.
http://www.siyabas.lk/files/SinhalaTamilKit_XP.exe
Sinhala Gini Nariya.