Step: 1
First of all, you need to install support for Windows complex script to view complex Asian characters like Bangla. You can download it from here (alt). Run it and install.
Step: 2
Install an Unicode Bangla font. You can download a nice Bangla font from here. Download it and copy that file into Control Panel > Fonts. After copying the font press F5 twice. It will refresh the font list Window.
Step 3
Configure you browser so that it uses your newly installed Bangla unicode font.
You are DONE after browser configuration.
For more in this issue under different Operating systems, visit this Wikipedia page or see this post.