I had the same problem. I "solved" it by deactivating the e-mail scanner in
my AVG Free Edition.

When I reply (by using Outlook 2007) to one of the e-mails where the body is
not rendered, the original message body is still missing when arriving at my
Hotmail account.

However, it's all still there when I use my webmail access to the IMAP
account to which Outlook 2007 is connected. So, the e-mail bodies themselves
have not been deleted/destroyed, Outlook 2007 is just not able to render them.

As already mentioned, with the e-mail scanner deactivated, new messages
render normally.