  Wind-blown bubbles, from those around massive O and Wolf-Rayet stars, to
superbubbles around OB associations and galactic winds in starburst galaxies,
have a dominant role in determining the structure of the Interstellar Medium.
X-ray observations of these bubbles are particularly important as most of their
volume is taken up with hot gas, 1E5 < T (K) < 1E8. However, it is difficult to
compare X-ray observations, usually analysed in terms of single or two
temperature spectral model fits, with theoretical models, as real bubbles do
not have such simple temperature distributions. In this introduction to a
series of papers detailing the observable X-ray properties of wind-blown
bubbles, we describe our method with which we aim to solve this problem,
analysing a simulation of a wind-blown bubble around a massive star. We model a
wind of constant mass and energy injection rate, blowing into a uniform ISM,
from which we calculate X-ray spectra as would be seen by the ROSAT PSPC. We
compare the properties of the bubble as would be inferred from the ROSAT data
with the true properties of the bubble in the simulation. We find standard
spectral models yield inferred properties that deviate significantly from the
true properties, even though the spectral fits are statistically acceptable,
and give no indication that they do not represent to true spectral
distribution. Our results suggest that in any case where the true source
spectrum does not come from a simple single or two temperature distribution the
"observed" X-ray properties cannot naively be used to infer the true
properties.
