Welcome to Seven Forums vadlamudibi.
For adjusting brightness just press F5 & F6 key's without Pressing Fun key.
If it still not work right click on the battery icon in lower right and choose
Power Options A window will popup there you can see your laptops power planes
Like 'Balanced' 'Power Saver' etc(it is good to put your laptop on Balanced).
Click on
Change plane settings. There You can adjust your laptops Brightness
Eather when pc is on battery or on AC. Adjust them As you like And that's it.
Edit: Hear you can Read a detailed tutorial on this:
Power Plan Settings
And hear you can read many more grate tutorials on
Power Management