On a Media Center enabled Windows like XP Media Center, Vista Ultimate, Vista Home Premium, 7 Ultimate, 7 Home Premium, pressing the green button on your MCE Remote will bring up Media Center. If you don't use Media Center this can be a problem (The same thing happens for the Guide button, Live TV button, Recorded TV buttons and some other buttons on all the variations of the MCE Remote)
With LM Remote KeyMap it is very easy to disable the native action of the MCE Remote green button (and all other buttons that seems unable to customize like the Power button).
1 - Install LM Remote KeyMap
If not already done, download LM Remote KeyMap and install it.
2 - Run LM Remote KeyMap and set up your MCE Remote
Open LM Remote KeyMap and in the main window choose "Customize your remote"

Choose to use a MCE Remote, click on the arrow buttons to flip through the remotes LM Remote KeyMap support until you get the MCE Remote, click on the remote to select it.

III - Disable the MCE Remote Green button native behavior
Once you selected the MCE Remote to use with LM Remote KeyMap you will be able to customize the native remote behavior, to do this click on the "Change remote settings" link.

This will open the remote settings dialog. In this dialog check the "Exclusive use of MCE remote (unplug / replug your remote to apply this feature) and click on the "Ok" button. This will disable the native behavior of the MCE green button (as well as the Guide, Live TV, Power etc...) buttons.

Note: you have to unplug and plug back the MCE Remote receiver. If your MCE Remote receiver is included in your PC (like with some Dell computers), exit completely from LM Remote KeyMap by clicking "Ok" in the various screens to save your changes and restart your PC. Also, if the LM Remote KeyMap Blaster service was not running, you will need to restart your PC (restart your PC two times to be sure that changes were applied and your MCE remote picked up the changes).
IV - Customize the MCE Remote Green button
You can now remap completely the MCE Remote green button. To do this from the LM Remote KeyMap main window select "Customize your remote", then in the "Application profiles" list click on the "Edit" link for the default profile (Note: you can create other profiles to target a specific application so as to have a different behavior for the MCE Remote green button for each application you usually use).

You are now in the application profile editor, you can add any actions to the MCE Remote green button: click on the green button on the remote picture, then click on the button "Add an action".

LM Remote KeyMap propose many types of actions :
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Keyboard shortcut: you can use any keyboard shortcut you want, LM Remote KeyMap will send it when you press on the MCE Remote green button
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Launching Application: you can launch any application you want when your press on the MCE Remote green button (you can launch applications in maximized, minimized state and even completely hidden)
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Switching focus: you can switch focus from an open application to another one
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And many more actions

