Sinhala letters are not combining

If your computer is a Windows XP computer, It probably does not have the Uniscribe font renderer installed. It is in the Windows XP CD (any version). We will show you how to get that done in a moment.

We think that you need to seriously think about updating your system to Service Pack 3. This is the final update for Windows. Read about it here:
Update Windows XP

However, you do not need the Service Pack upgdates to see properly joined Sinhala letters. The easy option is to go and install Unicode Sinhala version appropriate to your system:
Install Unicode Sinhala

The other method probably needs the Windows XP CD (any version). The procedure is as follows:
Control Panel -> Date, Time, Language, and Regional Options ->
Regional and Language Options (dialog box):
      Languages (tab)
      Place a ckeck / tick mark in the box in front of:
      "Install files for complex script..."
      Click OK (button)

See the image: