In a windows keyboard, is right ALT treated as AltGr?

• CTRL+ALT = AltGr.

Only in keyboard layouts which make distinction between Alt and AltGr. On these layouts:

• Applications natively born at Windows platform make nice disctinction between Ctrl+Alt and AltGr when there is a keyboard shortcut defined. For example, when Ctrl+Alt+E activates some shortcut mapping, AltGr+E still prints €. So in these apps, Ctrl+Alt and AltGr are not the same. But multi-platform apps (e.g. Eclipse IDE or some Firefox plugins) ignore this difference so AltGr+E launches Ctrl+Alt+E so you have no way to type the €. But this is rather a bug which hopefully developers fix, although developers using only US keyboard layout typically don't care and resist fixing this (as seen in Eclipse bug tracker). Should you create a Windows app, test this aspect to behave correctly.

• It is the right ALT not the left ALT.

Yes. Applies to keyboard layouts which make distinction between Alt and AltGr.

• Some keyboards have AltGr key.

Yes. Most of. On some netbooks, right Alt may be missing.

• Windows keyboard differentiate left ALT and right ALT.

All keys are differentiated from viewpoint of the operating system, i.e. left and right Shift, left and right Ctrl, left and right Alt, left and right Win are distinct keys although they are often interpreted in the same way – to give the user's hands comfort of pressing modifier which is reachable better. But for example some computer games allow to assign different action to left and right Shift key so when needed, they surely can act differently.