Hi,

It is quite easy really, as with everything, when you know how.

I wouldn't buy stickers - it's messy and looks cheap. just get a new keyboard with the typical Us 120+ keys but with Thai characters on them too. They cost 200-500 Baht and can be used as replacement keyboards for a desktop or a supplemental keyboard for a laptop.
Then, assuming you're using XP, go to 'Control Panel' and 'Regional and Language Options'. Click on 'Languages' and the first checkcox (it mentions Thai).
Then click on 'Details'; and 'Add'; choose your language(s); make sure you enable 'keyboard' as an input device. click 'OK'
Click on 'Language Bar' and enable it; Click on 'Key Settings' and adjust them, so ou can switch between English and Thai at the press of a button or click of the mouse.
Click 'OK' and 'OK' and you're done.

It sounds complicated, but when you go to see what I'm saying. you shouldn't have any trouble.

All the best,
Owen.