Computers and operating systems bundle fonts for most popular languages. Usually these fonts are Unicode compliant.
You may have seen web pages in Japanese, Chinese and Arabic render well on your computer without any additional fonts. Because most operating systems ship with fonts for these languages.
Sinhala, being a language used by only a small population of the world, has not been given very high priority by most proprietary vendors. Therefore, not all operating systems bundle Sinhala fonts.
If you are using such an operating system like Windows XP, you need to install a Sinhala Unicode font.
Some operating systems like Fedora Core 9 or Windows Vista bundles Sinhala Unicode fonts, so users of those don't need any additional fonts.