Minecraft won't load?

Make sure that :

u meet the system Requirements

And you bought the full version

Usually the demo doesn't open

is .NET Framework 4 important for my computer?

Well , most programs are using .NET Framework 4 so if you found a program that tells you don't have .NET Framework 4 , it means that you need it to use this program ,
ALL Programs that are created in Visual basic 2010 Opens with .NET Framework 4
It requires 1 GB , i don't recommend getting it UNLESS A program or your PC Tells you to download it