  The Swift Gamma-Ray Burst Explorer, launched on 2004 November 20, is a
multiwavelength, autonomous, rapid-slewing observatory for gamma-ray burst
(GRB) astronomy. On 2004 December 23, during the activation phase of the
mission, the Swift X-Ray Telescope (XRT) was pointed at a burst discovered
earlier that day by the Swift Burst Alert Telescope. A fading, uncataloged
X-ray source was discovered by the XRT and was observed over a period of about
3 hours, beginning 4.6 hours after the burst. The X-ray detection triggered a
VLT observation of the optical/NIR counterpart, located about 1.1 arcseconds
from the XRT position. The X-ray counterpart faded rapidly, with a power law
index of -1.72 +/- 0.20. The average unabsorbed X-ray flux 4.6-7.9 hours after
the burst was 6.5 x 10^{-12} erg cm^{-2} s^{-1} in the 0.5-10 keV band, for a
power-law spectrum of photon index 2.02 +/- 0.13 with Galactic absorption. The
NIR counterpart was observed at three epochs between 16 and 87 hours after the
burst, and faded with a power-law index of -1.14 +/- 0.08 with a
reddening-corrected SED power-law slope of -0.40 +/- 0.03. We find that the
X-ray and NIR data are consistent with a two-component jet in a wind medium,
with an early jet break in the narrow component and an underlying electron
index of 1.8-2.0.
