Remote Assistance - Vista requesting help from XP Home

Can Vista Home Premium send out a Remote Assistance request to and XP Home
machine?

My father bought a new computer with Vista. I have XP Home. I'd like to
take over his computer so he can see me set stuff up for him and solve some
issues he's having.

Both of us have a DSL modem connected to a router, then to our computers.
He sent me a request by saving the file and sending it via email. I opened
it, typed in the correct password but it would not establish a connection. I
get the following error message:

A Remote Assistance connection could not be established. You may want to
check for network issues or determine if the invitation expired or was
cancelled by the person who sent it.

Is there something we are not doing right?