On Windows 7 user files are saved in the Virtual Store rather than in the Program Files location.

For example this would be at:

On 32 bit Windows
c:\Users\username\AppData\Local\VirtualStore\Program Files\Pixarra\TwistedBrush\

On 64 bit Windows
c:\Users\username\AppData\Local\VirtualStore\Program Files (x86)\Pixarra\TwistedBrush\

In place of username you would use your Windows user name.

In sub directories of this location you will find your books (and pages/images), filter presets, clips, ArtSets and Scripts.