The only patch specifically for WinXP only (the one mentioned in this thread subject) lists 3 programs that have conflicts.
ap2011-17-07-002: Patch for slowdown with RAXCO PerfectDisk/Retrospect/Norton Ghost software, Windows XP only! (Free/Pro/IS)
Fairly clear to most people.
I’ve noticed no speed problem or other issues with Avast 6.0.1203. If downgrading clears some issue on your machine, then that’s what you had to do. It’s hard to foresee problems across all configurations. Even extensive beta testing cannot encounter what the final release will be installed alongside.