Problems with m7775.uk (with Xbox)

I bought, on the weekend, an HP Pavillion m7775.uk desktop machine.
The machine came preinstalled with Vista Home Premium.

I was, before now, using a MCE2005 PC with an Xbox 360 acting as a
media center extender in my living room.

What happens, with the HP m775 is that I can set up the connection
just fine using the add extender features of Vista Media Center, but
when I attempt to start the Media Center on the 360, it finds the
device, connects, goes to the main screen (ie. TV, Video, etc) and
then just freezes. Sometimes I can navigate the interface for up to
20 seconds before this happens, but it always freezes. The xbox is
still working (I can press the 360 guide button and the blade pops
out) but the Media Center Extender software just freezes. I have to
quit to the dashboard or resatrt the console.

I have tried everything over the weekend. I tried setting up a new
network with a new router. Same problems. I tried a direct
connection with a cross over cable to eliminate any network problems
and, yes, same problem. I even tried a new 360 but, sadly, same
problem.

At the same time of purchasing the HP, I also bagged a Sony Vaio Vista
laptop which works absolutely fine with the 360. I've seen other
people with the same problem on these forums and nobody has solved it
yet. HP support is terrible, Microsoft wouldn't speak to me as Vista
was preinstalled. PC World knew nothing, and Xbox support should have
called me back hours ago.

I have Vista drivers for all my components, and everything seems fine
and dandy. I can run the Media Center software perfectly on the HP PC,
but the moment I try to get my 360 extender connected to it, it screws
up.

Any help would be appreciated.

I have, for the record, tried disabling firewalls, anti virus
software, all start up programs, with no luck.