If you have the Sinhala pack for Windows XP (SP2) you should already
have an input method editor. You should see a වි icon on the system
tray (bottem right of screen)
Sinhala pack for Windows XP (SP2) is available at:
http://www.ucsc.cmb.ac.lk/ltrl/downloads/SinhalaXP0.42.zip
or at http://www.mrt.ac.lk/sinhala/SinhalaXP0.4.zip
The actual unicode character sequence however is (kayanna followed by
vowel sound of Oyanna). Thanlfully we need not worry about this if we
are using a proper IME.
If you are serious at all about using Sinhala language in computing (I
hope you are), either learn touch typing (I believe Sinhala touch
typing is easier than English) or get a nice keyboard with the Sinhala
letters pre-printed.
Hope this helped.
With regards,
Nalaka Jayasena