Questions about Metro UI:

1) There are many apps which do not have X button to close: How can I close these apps?
2) How can I launch apps in the Accessaries' folder or how to access the folder?

You don't need to close the Apps, they become suspended when you leave them. All the Applications in the Accessories are in Apps window, click search and click Apps in the right of the screen. HTH.

Suppose I start using Windows Media Player, and then lauch Whether. From then on, I can only flip between Metro tiles and the running Whether. The Media player keep playing unaffected. How can I turn off the Media Player?

How can I get to the "Apps windows"?

When you switch to Metro tiles, you can click the Desktop and switch off WMP. To access "Apps windows", move the cursor to the bottom left corner, click search then click Apps on the right side pop up Pane( second pic).

Thank you very much for your help.

You are welcome.