So you want to build your own wristwatch with Raspberry Pi? 314reactor has shared a guide that shows you how to do just that and run Windows 98 on it. You are going to need a touchscreen display, case, buttons, switch, Powerboost 500, and a few other parts to get started.
QEMU is the emulator chosen for this project. If you miss those good old days of constant blue screens of death, you may want to give this project a try. More info is available here.