Makes me wonder how well a updated Fujitsu U1010 would do today.
We used to have quite a few nice, small machines. The one of the Psion Series 5 was amazing for the size. I had an HP Windows CE machine that was only a little bigger and the keyboard was even better. Lack of USB, SD and Wi-Fi rather limits their usefulness nowadays ;-)
There are other Windows laptops where the screen folds back to provide a slightly heavier tablet, like the Lenovo Yoga 2 series and the HP11 x360.
Windows 8.1 tablets can support USB keyboards and full-size screens. Hook them up to a expanison hub and they can support multiple screens. I wouldn't recommend one as a desktop replacement, but then, I wouldn't recommend a MacBook Air as a desktop replacement either.