While browsing the net I came across some great Mini-PCs made
by a company called SD-Omega. Various flavours of microscopic PCs are
offered, from Celeron based book size systems to the much more powerful
Pentium4 based systems running at over 3Ghz!
I doubt these would make for a very practical PC system, they could make
great media PCs or when combined with a touch screen monitor and software like
GpsDrive, a great in-car navigation