Suggest Max Battery Laptop

Hi everyone.

Please suggest me a laptop which can go on 6 to 8 hours a day without recharge or AC Input, and

1. Should be light weight.
2. My uses would be restricted to Internet/Firefox/Google Books, MS Office 2010 Word and Wi-Fi Internet.
3. I can only work with Windows so Win 7 (no MAC stuff :blush:) and upwards (but no issues if a laptop comes cheap with DOS only and I can install Windows on it).
4. Definitely no games, no movies, no video/image playing or editing.
5. Possibility of Windows OS upgrade, though that is a low priority ... I'm happy with Win 7 Home.
6. I also don't care for a DVD drive since I won't be using it much and would be happy to use an external DVD/HDD for program installation/backup purposes etc. once in a while.

Thanks in advance. Further info, I have to carry it to various libraries for some research work.