I need to revert my current version of the QGIS Desktop Master back to 1ab5cdc (about 5 months ago). Anyone know how I can make this happen? I'm running windows XP
QGIS Master is intended to be nightly builds. Thats why nobody thought of keeping older releases for download. There are some older packages here: http://download.osgeo.org/osgeo4w/testing/release/qgis/qgis-dev/
but these are to young for your purpose.
Using the OSGEo4W setup installer, all your downloaded packages should have been saved somewhere on your system in \release\qgis\qgis-dev. But you have to take care for dependencies of other packages by yourself when reinstalling those.