grub2 - Couldn't boot Windows7 after installing 12.04 on a Dell N 5050 laptop

I installed 12.04(64bit) alongside Windows 7 (64bit) through a USB drive.

Earlier I couldn't load Ubuntu without a USB drive until I ran sudo grub-install --force /dev/sda2.

But now I can't load Windows 7 (loader)(on/dev/sda2) in GRUB. When I select that, it displays a black screen and then returns to GRUB.

Help me to solve this. I read "Installing GRUB 2" article but I cannot understand what I should do.