ATIH 2011 - Email backup asks for Windows credentials which aren't then accepted

Hi,
I've seen this issue mentioned in another thread but no help has been given so I'm trying a new one.

I'm running ATIH 2011 build 5519 on Windows XP Pro SP3 with all subsequent Microsoft updates. My problem is that I have tried to do a simple email backup. When I click on Backup Now, I am asked to enter my Windows credentials: Account name and Password. These I understand to be the Username and Password I use when I log onto Windows when I start my computer. However, when I enter these ATIH says, "The specified account name or password is incorrect". In my ATIH 2010, this worked but this is the first time I have tried it since upgrading to ATIH 2011.

My Windows account has administrative rights.

The problem occurs whether scheduling is turned on or off.

So I can't back up my emails. Can anyone help, please?

Thanks.

Rob