@Chris W et al.
Not sure of the other car reference so I will use my own:
If you go to a Ford dealer and want to buy a Fiesta, they will certainly sell you the new 2009 model, and they may have a couple of 2008 models still in stock, but if you want to but one of the old 90s square jobbies brand new, with warranty etc. then you are shit out of luck.
Microsoft can refuse to sell whatever they want. They are not forcing anyone to buy Vista, they are simply refusing to sell anything older*
Likewise resellers are not forcing anyone to buy Vista as such, they are simply offering XP as a cost modification, in the same way they might offer a different graphics card or memory combo. Even if they only supplied their computers with Vista then this is not something that lends itself to litigation, go somewhere else if you don't like their offerings.
Also, IIRC you can refuse to accept the bundled Windows license and demand a refund. You certainly can in the EU**, you will just screw up your warranty.
*Other than the XP downgrade, which has serious caveats and is only aimed at business anyway.
**Although I expect you would have lots of fuss actually doing this.