My colleague and I have published our calendars online and invited each other
to share them.

When he clicked on the link to access mine, it worked fine. When I clicked
on the link in the invitation email he sent me, it asks for a username and
password.

I have a live ID account, and have tried using that username (is that my
email address as it is when you log on to the live ID account online) and
password, and the dialogue box just pops up again.

It's infuriating!

thanks