I'm going to have to rely on my memory a bit here, but I'm pretty sure the location of icons on the screen is stored in the user's profile, and that the periodic rearrangement isn't because auto-arrange is turned on, but because the previous shut-down
caused Windows to believe the profile was bad, corrupted, etc. So, on reboot Windows looks for a previous profile, pops it and the "new" old icons arrangement and the result is that your hard-worked screen locations revert to some old one. If there was any
rhyme or reason to the icon arrangement I'd suspect Windows was doing the thinking on the rearrangement, but they're not suddenly all clustered to the left side of the screen, arranged alphabetically by name or location. On the other hand they don't appear
to be random either.
Lot's of Windows stuff gets its arrangement or visibility from "Most Recently Used" sorts-of stored arrangements, and when the latest MRU is perceived to be corrupted, the next most recent MRU is used. This stuff used to be fairly easy to maintain in XP, but
with Vista and beyond so much is now considered "Operating-System-critical" that you can't even go in and clean out the old ____ because Windows security doesn't think you know what you're doing, and won't let you delete files it thinks are system-critical.
With XP the workaround was to rename the file (which was permitted, regardless of how byzantine this sounds) whereas you couldn't have deleted the file even as admin. Once renamed, however, the new file name was easily deleted because its name was no longer
on the system-critical list of file names.
I know this doesn't help with the rearrangements, but at least you can stop thinking or worrying that it's auto-arrange or something like that. Profiles have been the bane of system administrators since Win95, and apparently continue to vex users 15 years later.
"The more things change, the more they stay the same..."
Paul