Hello friend. Welcome here!
What you ask is a newbie question and has already been discussed many times here. but i dont mind jottin them down once again.
For windows xp/2000, unless the user installs support for 'complex script' bengali text won't be displayed properly. however, for windows vista no such installation is necessary. and as far as 98 is concerned it is incapable of displaying unicode characters.
not just bengali to view any indic script say for example hindi an user has to install support for complex script in windows xp.
make no mistake - avro only helps you input bengali characters. it has no role whatsoever in how the text will be displayed. blame it on the os n browser.
give all this gyan to ur friends and they ll be able to view bangla their computers.

cheers!