The chaps from Microsoft were happy to inform the press that you can indeed run
Windows 7 on a netbook. They have demonstrated an Acer Aspire One netbook running
Windows 7 Ultimate version, of course in its Beta 1 iteration.
We are not sure how you install an beta operating system to a machine that doesn't have an optical drive, but you can probably put the unpacked iso on a USB key a tray. Of course, you can do it via an external USB DVD drive but this sounds simply too easy, and most people don't have these drive laying around anyhow.
Windows 7 is definitely not as responsive as it is on dual-core machines but it works rather nice on 1.6GHz clocked Atom based netbook. The chap didn't mention how much ram he had, but we believe it was the default 1GB, and in best case scenario we would be talking about 1.5GB.
You can see it in action here.