The duration of time it takes for Windows XP to actually finish booting up and take you to the desktop is rather inconsistent between different time-and-dates in libTAS (with half of the outcomes being a full 17 seconds longer!). I've documented the duration and outcome of some of these possible starting times in a spreadsheet here: https://docs.google.com/spreadsheets/d/1_j1zkYPPHR4jLLW3bybvnjsN6K7sV_vWZHLm7X5SwaE
Windows XP in general seems to be rather sensitive and unpredictable with lag. Simply moving the mouse around on screen seems to be able to affect the duration of loading times (often for the worse), and clicking on things the first frame they appear often seems to result in slower loading times than waiting a few frames beforehand. Without a better understanding of exactly what Windows does behind the scenes with managing tasks, the best solution here seems to be one of trial and error. Regarding avoidance of the mouse, I've found a rather comprehensive cheatsheet for keyboard shortcuts that may prove useful to others: http://www.shortcutmania.com/Windows-XP-Keyboard-Shortcuts-printable-cheatsheet.htm
CD loading times in particular seem to present a difficult hazard for TASing, as loading earlier savestates after PCem has started reading from the CD image seems to cause desyncs, and even strange errors from XP itself on some occasions:
Running the movie from the very start seems to give consistent timings without errors, but combined with XP's strange lag inconsistencies, it seems game installation in particular is likely to be a pain to optimise.