I say Thai keyboard but, read multilingual keyboard. I was googling for "Thai Keyboard" when I came across
Frontype. I know WinXP, Vista, Linux and probably Macs too have a screen keyboard built in but, this takes things a step further. Not just a multilingual keyboard but, a touch-typing tutor to boot! Within 5 minutes of me downloading the trial version, I bought the full version. I love it's simplicity. The trial version lasts for 7 days. Give it a try.