You can write Bengali in Windows 7.
Follow these steps to write Bangla in Windows 7
1. Click Start>Settings>Control Panel>Regional and Language Options
2. Click on the Keyboard and Languages tab.
3. Click Change Keyboards button.
4. It will open Text Services and Input Languages window.
5. Click Add button in the Installed serviecs pane.
6. Find Bengali (India) in the list. Click the (+) to expand the Bengali additional options and click (+) on the keyboard to expand further.
7. Put a check on Bengali box.
8. Click Ok button. This may ask installation CD. It will not ask it already installed.
9. Click Advanced Key settings tab in the Text services and Input languages window.
10. Choose To Bengali (India) in the List shown and click Change Key sequence button to configure Hot key for Bengali language.
11. Close all open window by clicking Ok buttons.
12. Now open any text editors like MS Word, MS Excel, MS Outlook, Wordpad, Notepad or any windows 7 application.
13. Press the key combination you set for Bengali to change the input language.
14. Now you can type in Bengali.
15. Press Key sequence for Between input languages (set in Region and Languages->Keyboards and Languages->Text Services and Input Languages->Advanced Key Settings) to switch back from Bengali language.