Is it possible to download from the net or customize the GUI, or form of a windows application made in Visual Studio?
I have tried to make the form better to look at, but i want cool or casual look on the form
Have a look at Krypton Toolkit for a free toolkit that will do basic "skinning" of your own winform application.
But, if you really want a next-gen UI to build your application with (MS based next-gen that is) - switch to Windows Presentation Foundation.
Now if you are asking how to change the UI of somebody else's application, then that is beyond the scope of this conversation, and I will just say you can't, easily.....