Vista - Remote assistance (Vista to XP)

I'm trying to offer remote assistance to another computer that has XP (sp2)
but I have Vista (ultimate).

I received the invitation file (.MsRcIncident extension) and tried to
connect. but I keep getting a dialog box saying the other computer must be
dis-connected from the internet or has closed remote assistance.

However, I know that the other computer is definitely connected to the
internet and has remote assistance open. Also, I know that the there is no
interference from firewalls.

Are Vista PC's able to connect to XP ones via remote assistance? Does remote
assistance on the other end really need to be opened (I thought that a window
would just pop-up automatically)?

Thanks.

I forgot to mention: I have made sure both computers have their settings to
allow remote assistance.

However the helpers PC (mine) has been set to not allow connections to this
computer under remote desktop (i believe this is different to remote
assistance).