Sleep/hibernate is the most logical solution.

Apart from trademark91's suggestion, it might be possible to:
- stick a VM on the fast booting Linux distro which automatically loads Windows from a snapshot
- boot by loading an image into RAM (since HDDs usually have fast sequential read but slow random read) and load off there (ie Diskless Angel)

I have an SSD and get a boot time of 15 seconds...

whaddayoutalkingabout is slow? @_@