<C-return> is undefined in nXML mode

Hello All,

I am not certain that this is specific to Windows, but since
I am on Windows, I will report it here.

C-Return (or M-Tab) is supposed to give me context-sensitive
completion of my XML document, but instead I get:

<C-return> is undefined.

I cannot use M-Tab because it is used by Windows to switch
between active Windows. How can I enable C-Return in nXML
mode?

Regards,
John Brown.