When it's at the post screen (before it tries to load Windows), press the F8 key every second or so to try to prompt the safe-mode extended menu. You'll know you're at the right menu if you see the option "Disable Automatic Restart on System Failure." Click on that selection, boot into Windows, and then write down and post the information that the blue screen is giving you. Such as the error code (Generally 0x00000somethingsomething) and the error description, such as "SYSTEM_SERVICE_EXCEPTION." Then, we can begin to help you.
"happy" = lappy ???
Never saw a happy blue screen, they are SAD for most people.
But yes, laptop, or desktop. First option is to perform steps digitalzom-b stated
Should have looked at which forum, so ofcourse it's a laptop - dummy me.
Added: You can also put the Windows installation disk in and select repair and it will try to replair the OS. NOTE: if you did not get a Windows 7 Installation/repair disk, you can just borrow a copy to do a repair.