I get this message when attempting to reply in both email accounts
on my system in Windows Live Mail:

"there was a problem opening the message"

 -- it opened for reading fine but won't open a sendable or forwardable copy, apparently.

This morning's bogus Windows 7 update crashed my Internet Explorer -- couldn't make ANY
web connections (inbound mail worked, though) so I told the system to revert to the prior good version.
That fixed my Internet Explorer problem but then I found the mail problem.
Not sure whether mail reply worked after the update but before reverting-- it worked yesterday.

I tried both "Repair Windows Essentials" and then a remove and re-install.  Strangely,
after re-install the same accounts were present, so account settings had not been
trashed (I expected them to) nor were emails deleted.

Anyway, still can't send on any account -- it doesn't get far enough to even create the
send message.  More than 4 hours wasted so far thanks to Windows Update!!
(I'm turning that off right now)