Since Windows 8 was first presented, there was a lot of fuss about whether existing Windows 7 applications will be compatible or not with the new operating system. The answer couldn't be any simpler, as developers are trying to enhance the current Windows experience and not bane it in any way. Some couldn't imagine how the new Metro UI will support hardcore applications like Fireworks or any other flash based programs. As I said, it's simple: it doesn't have to support anything.
The Metro UI is a simple panel between what the user sees and a normal Windows 7 desktop. Once you reach the old classic interface, any application can be opened as usual, and Windows 8 doesn't seems not to be any different from Windows 7. All existing applications will continue to work perfectly and, as a bonus, even the Windows 7 taskbar will be in place. The only palpable difference is the lack of the Start menu. Once the button is clicked, you will be returned to the Metro style.
To get things straight, all existing Windows 7 applications are compatible with Windows 8. This statement does not work both ways, because 8 comes with some apps that are developed in a Metro style, so they can be shown off on the Metro UI panel. Also, all W7 apps can be tuned (and they will surely be in the future) to support this new style and be accessible from the first screen.
Now, here's how to use a normal app in Windows 8:
First, you will have to reach the classic desktop, by closing Metro. This can be done by pressing the Desktop tile in Metro UI or by hitting the Windows + M shortcut key. Also, Alt + Tab should do.
Once on the classic menu is in place, navigate to the application you want to use and double-click its .exe file. Just like in W7.
Now, to launch a Metro UI based application, return to the Metro screen by pressing the Windows key and start typing. A list like the one below will appear where all installed applications will be sorted in alphabetical order. Clicking one of them will make it launch and that's it. Remember, while using the Metro Launchpad some apps may act differently. For example, there is no plugin support so that means if you start Internet Explorer you can't enjoy flash based websites.
Also, make sure that the screen resolution is above 1024x768, or some full-screen applications will fail to launch. This is part of the minimum system requirements.