Widescreen monitors - non-stretched desktops/apps

Hi folks!
I hope this is the right forum, and that no one thinks this question is completely insane.....

We have a new client, a small company of web designers. They want to test their site designs at various resolutions. That's normal, but the next part isn't.....

We're getting them some new PC's. These will be "bog standard" Dells with whatever graphics card Dell offers in its "deal of the week". So GPU drivers therefore are a complete unknown. Vista/XP will be dual boot options. They will also get lovely Dell widescreen monitors. Here's where things get weird.....

These guys will occasionally want to test at low resolution - simulating what the user with a 15" monitor (or smaller) and 800x600, 1024x768 and so on, stepping through several combinations of screen size/resolution would see. Their goal is to make sure the site looks the same on anything from an eeePC/netbook all the way up to a 480" Mega Plasma monitor in Times Square (OK, the last point is just a little bit of an exaggeration )

They are willing to deal with the ugly black borders of a small display on a big monitor - in fact specifically want that instead of stretching the desktop/browser window all the way to the dimensions of their monitors. My questions to the community therefore are;
a) Can this be done natively somehow in XP/Vista? (If it is, I'm completely unfamiliar with any way of going about such a thing)
b) If this can't be accomplished natively, does anyone know of the existence of a third party 'app which could setup the simulation situation described above?

Suggestions, Ideas, comments, all eagerly welcomed.

Regards & TIA!
./porky