how to remove windows multiple boot option?

 You need to fix your MBR (Master boot record).

Option One: Open Start-Run and enter msconfig Click the tab "BOOT.INI" The contents of the boot.ini file are shown and a number of configuration settings are available. Under Operating systems, delete the one you don't want. Be careful not to delete the wrong one.

Option Two: Right-click on "My Computer" and choose "Properties" from the context menu. The System Properties dialog box will open. In the section "Startup and Recovery" click the button "Settings". To delete the unwanted operating system from the boot display menu, use the "Edit" button on the right of the entry.

Option Three: If the OS is Windows XP Professional, it has a command line utility for manipulating boot.ini called bootcfg.exe.
Click start, type CMD, click OK and you are presented with the command prompt. Type bootcfg /? It will list the description and parameter list. Once again, you can delete the unwanted OS.

The option you choose depends on your preference but they all do same thing. But before you decide to mess with boot.ini, try this: Boot the system with windows CD, select the option to repair windows, type the password, type fixmbr. After that type fixboot. Restart the system and hopefully it will fix the problem without tinkering with boot.ini

If you have more question, let me know.

how to fix windows 8.1 brightness problem

I have successfully found the solution for the problems you guys are facing!
I too had the same problem before, as it is because of the lack of display drivers of your device!
Steps to fix the problem:-

1.Right click on the MY COMPUTER icon on the desktop.

2.Select the Manage option.

3.Now select the Device Manager from the left pane on the screen.

4.Select the Display Adapters pane from the list on the center of your screen.

5.It will show you the installed display driver on your system. Right click on the driver and select the option update drivers.

Once the drivers are updated to the required drivers for your system it will automatically enable your brighteness option and hotkeys on your keyboard.
Surely works!