OpenOffice.org

All Open Office
versions after the version 2 provide Sinhala Unicode support. Open
Office works perfectly not only on all GNU Linux flavors, but also on
Windows
XP and Vista.

Microsoft Office 2003

If the Sinhala Kit is installed for Windows XP, you can use all applications in Microsoft Office 2003. However ordering (alphabetically) is not possible. A list of problems reported in MS Office 2003 applications are listed here.

Sinhala Unicode works much better on MS Office 2003 when it is run on Windows Vista

Microsoft Office 2007  

MS Office 2007 works correctly with Sinhala Unicode on Windows XP and Windows Vista

However ordering (alphabetically) is not possible; but ICTA and Microsoft are working to provide this support for Windows Vista in 2007.