Did you try any of the shops on my list
http://www.adelaidecomputerstores.com/ ?
If they don't build them them selves there's always a chance they'll know of people who do.
Another way, (and probably cheaper) is to go to a DAW builders site. Look at their machines, show their specs to someone who builds computers, they can source the parts and build it for you
for considerably less. As far as tweaking and configuring there really isn't very much that needs to be done with windows 7 compared to earlies versions of windows.
The first thing I do is change my user account settings and the other stuff I can't remember can be googled easily enough and is very easy to do.
Just a thought.
Steve